An inhibitor from placenta specifically binds urokinase and inhibits plasminogen activator released from ovarian carcinoma in tissue culture.

An inhibitor present in placenta and released in placental tissue culture forms specific complexes with each of two molecular forms of urokinase. Autoradiography demonstrated that the inhibitor shifted the electrophoretic position of 125I-labelled urokinase. It did not change the migration of diisopropyl-fluorophosphate-inactivated 125I-labelled urokinase, thereby indicating complex formation dependent on active serine site in urokinase. The inhibitor had a strong neutralizing effect on the plasminogen activators released from human ovarian carcinoma in tissue culture. The placental inhibitor might prove useful in inhibiting the fibrinolytic process necessary for proliferation of tumour vessels.

Identity between the placental protein PP10 and the specific plasminogen activator inhibitor of placental type PAI-2.

The highly specific plasminogen activator inhibitor of placental type, PAI-2, occurs in the placenta in a low molecular mass form of 46.6 kDa, and in pregnancy plasma in a (possibly glycosylated) high molecular mass form of 60 kDa. Extensive knowledge is available about the functional properties of PAI-2 as a plasminogen activator inhibitor and about its molecular biology and regulation. Of the several placenta proteins (PP) isolated, one of them, PP10, has a molecular mass of 48 kDa and its occurrence in malignancy and in complications during pregnancy has been the topic of a number of studies, though its properties and physiological significance are unknown. The present findings constitute evidence of immunological identity between PP10 and PAI-2. The sections of the amino acid sequence of PP10 analysed here were found to have identical counterparts in the sequence of the low molecular mass form of PA1-2, but in several preparations PP10 was found to occur in an inactive two-chain form due to cleavage of an Arg-Thr bond, the two peptide chains being linked to each other by a disulphide bridge. The cleavage site is identical to that observed in the reaction between PAI-2 and urokinase. The results make it possible to coordinate and correlate the findings of many separate studies and our own observations on PP10 and PAI-2.

Urokinase binds to a plasminogen activator inhibitor type-2-like molecule in placental microvillous membranes.

Placental microvillous membranes exhibited saturable binding of urokinase-type plasminogen activator with plateau achieved by 30 min at 4 degrees C and 10 min at 37 degrees C. The binding was essentially irreversible. The capacity was about 8 pmol urokinase per mg membrane protein. Half-maximal displacement of 125I-labelled urokinase was achieved with about 1.0 nM unlabelled urokinase when using 75 micrograms membrane protein/ml. 125I-labelled urokinase did not bind when treated with diisopropylfluorophosphate to block the catalytic activity. Single-chain urokinase (prourokinase), devoid of catalytic activity, did not bind. Catalytically active tissue-type plasminogen activator did compete with 125I-labelled urokinase for binding although less efficiently than urokinase. Binding activity remained in the 100,000 x g pellet after treatment of the membranes with 3 M KCl, alkaline stripping at pH 12 or extraction by the detergent Triton X-100. The binding was essentially blocked by antibodies against plasminogen activator inhibitor-type-2 (PAI-2). Sodium dodecyl sulfate polyacrylamide gel electrophoresis of solubilized membranes with bound 125I-labelled urokinase showed that the urokinase-PAI-2 complexes largely migrated in fractions corresponding to a very large Mr although no clearly defined peaks were observed. It is suggested that PAI-2 occurs in a form anchored to syncytiotrophoblast microvilli, possibly to the cytoskeleton.

Plasminogen activators and plasminogen activator inhibitors in blood and tumour fluids of patients with ovarian cancer.

We quantitated urokinase and tissue plasminogen activator (u-PA, t-PA), plasminogen activator inhibitor 1 and 2 (PAI-1, PAI-2), and fibrinolytic activity in peripheral blood (PB), tumour blood (TB), peritoneal/ascitic fluid (PAF) and cystic fluid (CF) from 104 patients with benign and 36 patients with malignant ovarian tumours, and in peripheral blood from 62 healthy controls. PB levels of u-PA were higher in patients with benign and malignant tumours than in controls. High concentrations of u-PA were found in CF, but not in TB, suggesting that u-PA is released by the tumour tissue, but not by the tumour vasculature. PB levels of t-PA were higher in both tumour groups than in controls. Increased levels of t-PA were found in TB, but not in CF, indicating that t-PA is released by the tumour vasculature, but not by the tumour tissue. PB levels of PAI-1 were higher in patients with both benign and malignant tumours than in controls. High levels of PAI-1 were present in both TB and CF from malignant tumours, suggesting that PAI-1 is released from the tumour vasculature as well as the tumour tissue. Elevated concentrations of PAI-2 were found in CF, but not in TB, indicating release from the tumour tissue, but not from the vasculature. High levels of t-PA, PAI-1 and PAI-2 were found in PAF of malignant tumours, and resorption from this compartment may explain elevated PB levels in patients with ascites. None of the PAs/PAIs proved useful as a PB marker for detection of early stage ovarian cancer. However, an index based on PAF levels of t-PA and PAI-1 discriminated between malignant and benign ovarian cysts in the absence of ascites. In addition, our study stresses the importance of including patients with benign tumours as well as healthy controls when markers for malignant tumours are evaluated.

Cellular localisation in placenta of placental type plasminogen activator inhibitor.

A specific plasminogen activator inhibitor is known to occur in placenta and in pregnancy plasma. Immunohistochemical methods with polyclonal and monoclonal antibodies against the inhibitor were used for its localisation in term placentas. Immunoreactive material was found in the trophoblastic epithelium. It was absent in the stroma of the chorion villi.

Purification of a specific placental plasminogen activator inhibitor by monoclonal antibody and its complex formation with plasminogen activator.

A monoclonal antibody of IgG2a-type was obtained against a specific fast acting plasminogen activator inhibitor found in placenta. The placental inhibitor was purified by affinity chromatography using the monoclonal antibody and additionally in a FPLC-system. A strong complex formation was found between the inhibitor and urokinase and also with the two-chain form of plasminogen activator of the tissue-type. A weaker complex was found between the placental inhibitor and the one-chain form of the tissue-type activator.

Fluid characteristics of benign ovarian cysts: correlation with recurrence after puncture.

OBJECTIVE: To determine whether concentrations of gonadal steroids and fibrinolytic indices in fluid from benign ovarian cysts can discriminate between functional and neoplastic cysts and predict recurrence after ultrasound-guided puncture. METHODS: Concentrations of gonadal steroids and components of the plasminogen-activating system were measured in cyst fluid obtained at ultrasound-guided puncture of 96 ovarian cysts and were related to subsequent cyst recurrence. In 83 patients who had surgery for benign ovarian cysts, components of the plasminogen-activating system in the cyst fluid were correlated with the histopathologic diagnosis. RESULTS: Higher levels of plasminogen activators and lower levels of inhibitors were found in those 54 cysts that recurred after puncture and in cysts with low levels (below 2000 pmol/L) of estradiol (E2). This enzyme-inhibitor balance resulted in high fibrinolytic activity. In contrast, cysts with high E2 levels (above 2000 pmol/L) had lower levels of activators, higher levels of inhibitors, and virtually no fibrinolytic activity. A high E2 concentration in cyst fluid was the single best predictor of no recurrence after puncture. Sixteen of 18 cysts in postmenopausal women recurred, and all had low levels of E2. However, an index based on cyst fluid volume and concentrations of E2 and urokinase predicted recurrence even better. A high concentration of urokinase in the fluid correlated with neoplastic histology of the cysts obtained at laparotomy. CONCLUSION: The fluid content of ovarian steroids and plasminogen activators and inhibitors is related to histopathology and recurrence after puncture of benign ovarian cysts. Puncture and assay of these components may minimize surgery on functional cysts.

Fibrinolytic factors in endometriotic tissue, endometrium, peritoneal fluid, and plasma from women with endometriosis and in endometrium and peritoneal fluid from healthy women.

OBJECTIVE: To determine whether there is a difference in fibrinolytic compounds in endometriotic tissue, endometrium, peritoneal fluid (PF), and plasma from women with endometriosis and in endometrium and PF from healthy women. DESIGN: Prospective study. SETTING: Two university clinics. PATIENT(S): Regularly menstruating women with and without endometriosis. INTERVENTION(S): Tissue samples, PF, and blood were collected at surgery performed for clinical reasons. MAIN OUTCOME MEASURE(S): The antigen concentrations of plasminogen activators and plasminogen activator inhibitors (PAls) in tissue homogenates, PF, and plasma were assayed by ELISA. RESULT(S): The concentrations of urokinase plasminogen activator (u-PA) and PAI-1 were higher in endometrium from women with endometriosis than in endometrium from controls and even higher in endometriotic tissue than in endometrium from both groups. In PF, the concentration of PAI-2 was higher in women with endometriosis than in controls. CONCLUSION(S): The high concentrations of u-PA and PAI-1 in endometrium from women with endometriosis might facilitate implantation of endometrial cells and the high concentration in endometriotic tissue might contribute to their invasive growth. The inflammatory reaction may contribute to the high concentration of PAI-2.

Placental and decidual u-PA, t-PA, PAI-1 and PAI-2 concentrations, as affected by cervical dilatation with laminaria tents or Hegar dilators.

The effect of cervical dilatation on PA and PAI concentrations was compared in a laminaria tent group and a mechanical dilatation (control) group. In both groups, u-PA and t-PA were found to be present both in decidual and placental tissue. Values for PAI-1 in the decidua were higher in the laminaria tent group than in controls, and those for PAI-2 in the placenta were lower. These findings provide support for the contention that cervical dilatation with laminaria tents may result in partial detachment of the placenta.

PIP: It has been suggested that placental detachment is associated with increased fibrinolytic activity in the villous tissue. There are also indications that cervical dilatation with laminaria tents contributes to detachment of the placenta. In this study, the decidual and placental concentrations of plasminogen activators (PAs) and plasminogen activator inhibitors (PAIs)--key components of the fibrinolytic system--were measured in 69 1st-trimester abortion patients, 35 of whom were dilated by laminaria tent and 34 of whom were in Hegar dilator group. PAI-1 concentrations were significantly higher in the laminaria tent group than in controls, while PAI-2 values were significantly lower. The inverse relationship between urokinase-related PAs (u-PAs) and PAI-1 in the decidua was largely the same in both groups, while there was a significant difference between groups in the relationship between u-PA and PAI-2. Decidua in the laminaria tent group further differed from controls with regard to the relationship between blood vessel-related PA (t-PA) and PAI-2., but not between t-PA and PAI-1. In placental specimens, the 2 groups differed in terms of the relationship between u- PA and PAI-1 concentrations; inhibitor activation decreased with increasing u-PA values in the laminaria tent group. Laminaria tent subjects showed an inverse relationship between placental concentrations of u-PA and PAI-2, while Hegar dilator controls demonstrated a horizontal regression line. Finally, relationships between t-PA and both PAI-1 and PAI-2 placental concentrations were opposite in the 2 study groups. These findings support the observation that laminaria tent cervical dilatation contributes to placental detachment.

Plasminogen activator inhibitors from placenta and fibrosarcoma cells are antigenically different as evaluated with monoclonal and polyclonal antibodies.

Plasminogen activator inhibitor (PAI) purified from human placenta was compared to PAI purified from conditioned cell culture fluid of the human fibrosarcoma cell line HT-1080. The two inhibitors had a similar mobility (Mr approximately 50,000) in sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). Purified placental inhibitor revealed 2 major and 1 minor Coomassie blue stainable bands, while the fibrosarcoma inhibitor appeared as one band. By immunoblotting analysis both monoclonal and polyclonal antibodies against each of the inhibitors showed reaction with the inhibitor against which they were raised, but not cross reaction with the other inhibitor. Similar results were obtained, when antibody binding was tested by ELISA with the inhibitors coated on the solid phase. HPLC fingerprint patterns of cyanogen bromide fragments of the two inhibitors were different. The inhibitory activity of the placental PAI was decreased by a factor of 3 after incubation with SDS, while that of the fibrosarcoma PAI was increased by a factor of 30. It is concluded that the two inhibitors show no detectable common antigenic determinants and most likely are products of different genes.

Plasminogen activators in the human endometrium, cellular origin and hormonal regulation.

Endometrial tissue explants in culture were found to release urokinase-type plasminogen activator (u-PA) and tissue-type plasminogen activator (t-PA). In order to identify their cellular origin and possible hormonal regulation, enriched cultures of glandular epithelial cells and stromal cells were prepared from fresh endometrium, and the cultures treated with hormones. Both epithelial and stromal cell cultures were found to secrete u-PA and t-PA. Treatment of epithelial cell cultures with oestradiol, progesterone and DH-testosterone had no effect on the secretion of t-PA or u-PA. In stromal cell cultures, on the other hand, the secretion of u-PA was significantly reduced after treatment with progesterone, whereas oestradiol and DH-testosterone had no effect. This reduction of u-PA antigen in the tissue culture medium did not result from a reduction of the relative level of u-PA mRNA in the cells, suggesting that the synthesis of u-PA was not reduced. Alternatively, an increased clearance of u-PA by the cells from the medium may explain the reduction. This in vitro observation probably reflects the in vivo reduction of u-PA in endometrial secretion during the secretory phase.

Transdermal estrogen replacement therapy: beneficial effects on hemostatic risk factors for cardiovascular disease.

OBJECTIVES: To assess the effect of estrogen replacement therapy on hemostatic risk factors for cardiovascular disease (CVD) in postmenopausal women during 2 years of treatment. METHODS: In an open prospective study, patients (n = 42) were investigated before and during 2 years of treatment, and compared to an untreated postmenopausal control group (n = 18) followed during the same period, healthy premenopausal women (n = 20) being included as a reference group for premenopausal values. The patients underwent treatment with transdermal 17 beta-estradiol (E2) (50 micrograms/24 h), oral medroxyprogesterone acetate (5 mg/day) being added for 12 days every second month. RESULTS: After 2 years of treatment there was a significant increase in t-PA antigen (P = 0.01) and a significant decrease in F VII antigen (P = 0.01). PAI-1 antigen concentrations decreased slightly. Fibrinogen concentrations were already significantly decreased at 3-month follow-up (P = 0.01), and were still low after 2 years. By contrast, at 2-year follow-up the postmenopausal control group manifested significant increases in F VII and PAI-1 antigen and slight increases in fibrinogen, which resulted in significant differences between patients and controls. Regression analysis showed the increase in the serum estradiol concentrations to be inversely correlated to the decreases in the plasma concentrations of F VII antigen (r = -0.34, P = 0.001) and fibrinogen (r = -0.35, P = 0.001). There were no changes in AT III or protein C in any group. CONCLUSIONS: The increase in serum estradiol concentrations due to replacement therapy did not adversely affect the studied components of the fibrinolytic and protein C defense system against thrombosis, and resulted in beneficial decreases in F VII antigen and fibrinogen. These findings may help to explain the beneficial effects of estrogen replacement therapy in terms of protection from cardiovascular disease.

Passage of the menopause is followed by haemostatic changes.

The passage of the menopause has been reported to be followed by a steadily increasing risk of cardiovascular disease (CVD). Changes in the concentrations of certain coagulation factors and fibrinolytic components are considered risk factors for CVD. We evaluated the differences in some of these variables between a premenopausal group (A) (n = 28) and two postmenopausal groups, one of women less than 18 months past the menopause (B) (n = 28), the other of women more than 18 months past the menopause (C) (n = 21). The variables measured were serum oestradiol content, plasma antithrombin III (AT III) activity, protein C activity and the plasma concentrations of tissue type plasminogen activator (t-PA) antigen, plasminogen activator inhibitor (PAI-1) antigen, and fibrinogen. As compared with the premenopausal women (group A), group C showed significantly higher values for AT III and protein C activity and for t-PA and PAI-1 antigen; and group B and C both showed significantly higher fibrinogen concentrations. This probably means that haemostatic balance was maintained in the postmenopausal women, although the increased concentrations of fibrinogen and PAI-1 might constitute risk factors for the development of cardiovascular disease.

Rebound increase of PAI-1 following local intra-arterial rt-PA infusion, a possible cause of reocclusion.

We studied systemic changes in the plasminogen activating system induced by local intra-arterial infusion of recombinant t-PA (rt-PA) in ten patients with arterial occlusion. The arterial infusion of rt-PA resulted in an immediate increase of t-PA activity and antigen in venous plasma. Concomitantly the plasma was depleted of PAI-1 activity with a moderate decrease of PAI-1 antigen, presumably due to immediate complexing of rt-PA by PAI-1 and gradual elimination of the rt-PA/PAI-1 complex from the circulation. Activation of plasminogen in the circulation was reflected by the occurrence of fibrinogen/fibrin degradation products. This rapid systemic activation of the fibrinolytic system at local intra-arterial administration of rt-PA might be due to the opening of small collaterals or backward flow through open arteries, allowing rt-PA to enter the general circulation. The disappearance of detectable PAI-1 activity during rt-PA infusion was followed by a rebound effect with a significant increase of PAI-1 antigen. As this increase may have contributed to the reocclusion which occurred in four patients, it might be advisable to replace heparin by rt-PA at a lower dosage in the immediate phase following rt-PA infusion.

Fibrinolysis in the peritoneal fluid during adhesions, endometriosis and ongoing pelvic inflammatory disease.

The concentrations of the specific activators (u-PA and t-PA) and the specific inhibitors (PAI-1 and PAI-2) of the fibrinolytic system were analyzed in the peritoneal fluid in women suffering from intra-abdominal adhesions, endometriosis or pelvic inflammatory diseases (PID). Peritoneal fluids were collected from ten women in whom a laparotomy was performed and an additional 108 in whom a laparoscopy was carried out. In comparison with the normal control patients all activators and inhibitors were significantly increased in cases of PID and when a second-look laparoscopy was performed one week after laparotomy with adhesiolysis. At laparoscopies, when adhesions were verified, u-PA in the peritoneal fluid was significantly increased and in cases of endometriosis PAI-2 was significantly reduced. The start of a laparotomy in order to remove adhesions, initiates a process, resulting in a significant increase of PAI-2 antigen in the pelvic fluid. The results imply that the fibrinolytic system is comprehensively activated in the peritoneal cavity during ongoing inflammatory reaction, and after adhesiolysis. The increase in plasminogen activators in the peritoneal fluid in established cases of pelvic adhesions or endometriosis may indicate that the fibrinolytic system is continuously active to inhibit the further formation of adhesions.

Localization of plasminogen activators and plasminogen-activator inhibitors in human gingival tissues demonstrated by immunohistochemistry and in situ hybridization.

The plasminogen-activating system plays an important part in tissue proteolysis in physiological as well as pathological processes. Plasminogen activators u-PA (urokinase) and t-PA (tissue) as well as the inhibitors PAI-1 and PAI-2 are present in gingival crevicular fluid in concentrations significantly greater than in plasma. This fact, and the finding that the concentrations of t-PA and PAI-2 are higher in areas with gingival inflammation, indicate local production of these components. The present study describes, by means of in situ hybridization and immunohistochemistry, the localization of the plasminogen activators and their inhibitors in gingival tissues from patients undergoing periodontal surgery. t-PA mRNA and t-PA antigen were primarily found in the epithelial tissues, predominantly in the sulcular and junctional regions, although occasionally in the oral epithelium and in blood vessels of the connective tissue. u-PA and u-PA-receptor signals were seen in single cells within the junctional and sulcular epithelia and adjacent to blood vessels close to the junctional epithelium, but rarely in the oral epithelium. Similar to t-PA, the predominant location of PAI-2 mRNA was the gingival epithelia. In the junctional and sulcular epithelia, PAI-2 mRNA was seen throughout the thickness, while in the oral epithelium the strongest signals were seen in stratum granulosum and stratum spinosum. PAI-1 mRNA was invariably found in the connective tissue associated with blood vessels. The present study confirms earlier indications of local production of plasminogen activators and their inhibitors in gingival tissues. In addition, the results demonstrate that t-PA and PAI-2 in these patients are produced predominantly in the epithelial tissues. Furthermore, the presence of t-PA and PAI-2 seems to be most pronounced in the areas likely to be subjected to bacterial assault.

Increasing expression of tissue plasminogen activator and plasminogen activator inhibitor type 2 in dog gingival tissues with progressive inflammation.

Urokinase and tissue-type plasminogen activators (u--PA and t--PA) are serine proteases that convert plasminogen into plasmin, which degrades matrix proteins and activates metalloproteinases. The PAs are balanced by specific inhibitors (PAI--1 and PAI--2). Local production of t--PA and PAI--2 was recently demonstrated in human gingival tissues. The aim now was to investigate the production and localization of t--PA and PAI--2 in gingival tissues from dogs in three well-defined periodontal conditions; clinically healthy gingiva, chronic gingivitis and an initial stage of ligature-induced loss of attachment. At the start of the experiment the gingiva showed clear signs of inflammation. Clinically healthy gingiva were obtained after 21 days period of intense oral hygiene. Attachment loss was induced by placing rubber ligatures around the neck of some teeth. Biopsies were taken from areas representing the different conditions and prepared for in situ hybridization and immunohistochemistry. In clinically healthy gingiva both t--PA mRNA and antigen were expressed in a thin outer layer of the sulcular and junctional epithelia. No t--PA signals or staining were seen in connective tissue. Both mRNA signaling and immunostaining for t--PA were stronger in chronic gingivitis. In areas with loss of attachment, t--PA mRNA as well as antigen were found in the sulcular and junctional epithelia to a similar degree as in gingivitis. Occasionally the connective tissue was involved, especially in connection with vessels. PAI--2 mRNA was seen in a thin outer layer of the sulcular and junctional epithelia in clinically healthy gingiva, but no signals were seen in connective tissue. PAI--2 antigen was found primarily in the outer layer of the sulcular and junctional epithelia. Some cells in the connective tissue were stained. In gingivitis, PAI--2 signals were mainly found in the same locations, but more intense and extending towards the connective tissue. Immunostaining was seen in the outer half of the sulcular and junctional epithelia as well as in the upper part of the connective tissue, close to the sulcular epithelium. In sites with loss of attachment, PAI--2 mRNA was found throughout the sulcular and junctional epithelia, as was the antigen, which stained intensely. No PAI--2 mRNA was seen in connective tissue; the antigen was found scattered, especially near vessels. This study shows that the expression of both t--PA and PAI--2 increases with experimental gingival inflammation in the dog, and furthermore, the two techniques demonstrate a strong correlation between the topographical distribution of the site of protein synthesis and the tissue location of the antigens for both t--PA and PAI--2. The distribution correlates well with previous findings in humans.

The effect of cervical dilatation by laminaria tent in first trimester legal abortions on blood loss related to fibrinolytic activity in the decidua and placenta.

Increased blood loss (BL) has been reported after cervical dilatation by laminaria tent in legal abortions. The BL was measured in 72 women in whom first trimester legal vacuum aspiration was performed. The cervical canal was dilatated by laminaria tent in 37 patients, and mechanically with Hegar dilators in 35 patients. BL was studied in relation to the plasminogen activators (u-PA, t-PA) and the plasminogen activator inhibitors (PAI-1, PAI-2) in the decidua and placenta. There was no significant difference in BL between the two groups. Decidual PAI-1 concentrations were significantly higher in the laminaria tent group than in the Hegar dilator group. An inactivation of u-PA by PAI-1 might explain the lack of increase in BL among the laminaria tent group.

Specific plasminogen activator inhibitor of placental type PAI 2 occurring in amniotic fluid and cord blood.

Plasminogen activator inhibitor of placental type (PAI 2) occurs in a high molecular weight form (HMW) of 60 kd and a low molecular weight form (LMW) of 48 kd. Amniotic fluid and umbilical cord and maternal term plasma were examined for presence of PAI 2 using an enzyme-linked immunosorbent assay sandwich technique, purification on an affinity column (monoclonal antibodies coupled to Sepharose 4B), and immunoblotting. The median values of the concentrations of PAI 2 were 89, 13, and 75 micrograms/L, respectively. The HMW and LMW forms were found at about the same concentrations in amniotic fluid, but the LMW form predominated in cord blood, and the HMW form in maternal blood. The physiologic function of PAI 2 in the child is probably to prevent bleeding during the hazards of parturition.

Isolation of a new specific plasminogen activator inhibitor from pregnancy plasma.

A specific plasminogen activator inhibitor was isolated from the plasma of pregnant women by matrix-bound, cross reacting monoclonal antibodies against placental plasminogen activator inhibitor. The pregnancy plasma plasminogen activator inhibitor (PP-PA-I) was found to be immunologically different from the inhibitor produced by endothelial cells. Its molecular weight was 70 000 daltons. It formed complexes with urokinase (u-PA) and with plasminogen activator of the tissue type (t-PA), similar to those formed by the placental plasminogen activator inhibitor (PI-PA-I). It did not inhibit plasmin. For measuring PP-PA-I, an enzyme-linked immunosorbent assay (ELISA) was designed using monoclonal and polyclonal antibodies against the placental inhibitor. Concentrations of PP-PA-I increased successively during pregnancy, and fell sharply after delivery. This inhibitor could not be detected in normal non-pregnancy plasma. The results indicate that the inhibitor isolated from pregnancy plasma is responsible for the depressed fibrinolytic activity during pregnancy, and that the placenta is the source of the inhibitor.