Mechanisms of chromium isotope fractionation and the applications in the environment.

Chromium (Cr) is a toxic heavy metal that gives rise to environmental pollution and human risk. Chromium stable isotopes have a wide range of applications in both environmental field and earth science field. In this contribution, we focus on the application of the Cr isotope in both tracing pollution sources and monitoring Cr(Ⅵ) pollution. Meanwhile, we also provide a description of the main influencing factors controlling Cr isotope fractionation, chromium isotope analytical methods, and terrestrial Cr release. Chromium isotope tracing of contaminant sources is a new application method, it has a tremendous advantage in searching for the source of Cr pollution, which has not been covered in previous reviews. At the end of the article, the current status of Cr isotope applications in the paleo-environment is explained. Although there are still some uncertainties in practical applications, chromium isotope system shows great promise in the environmental aspects.

Factors Affecting the Robustness of Data Inversion for Stable Isotope Measurement Using the Double Spike Method: Insights from Chromium Isotope Analysis.

Stable isotope ratios are widely used to solve environmental, geological, medical, and forensic problems. The double spike technique is considered to be one of the most robust and efficient methods to correct for instrumental mass bias and isotopic fractionation that may occur during sample preparation. However, various hidden errors can arise from data processing and have been largely overlooked in previous studies. Several of these hidden errors were investigated in this work using measurement and synthetic data. Double spike inversion of chromium isotope raw data from 1116 natural samples demonstrated that averaging raw isotope ratios before double spike inversion can add significant errors to inverted isotope values, and such errors can be 1.5 times larger than the true analytical precision. Synthetic data were used to investigate the errors on inverted Cr isotope data caused by spike:analyte ratio and Fe-Ti-V interferences, and the following threshold values are recommended to minimize such errors: 54Crspike/52Crsample ratio greater than 0.5, 56Fe/52Cr less than 0.2, 49Ti/52Cr less than 0.04, and 51V/52Cr less than 1. Sample preparation can potentially lead to large errors in inverted Cr isotope data if preparation-induced isotope fractionation deviates from the exponential law used in the double spike inversion, but such errors can be minimized by achieving >70% Cr yield. Our findings provide important insights for the double spike inversion procedure and assessing the reliability of inverted isotope data for not only the chromium isotope system but also other elements commonly analyzed using the double spike technique.

Atmospheric oxygenation three billion years ago.

It is widely assumed that atmospheric oxygen concentrations remained persistently low (less than 10(-5) times present levels) for about the first 2 billion years of Earth's history. The first long-term oxygenation of the atmosphere is thought to have taken place around 2.3 billion years ago, during the Great Oxidation Event. Geochemical indications of transient atmospheric oxygenation, however, date back to 2.6-2.7 billion years ago. Here we examine the distribution of chromium isotopes and redox-sensitive metals in the approximately 3-billion-year-old Nsuze palaeosol and in the near-contemporaneous Ijzermyn iron formation from the Pongola Supergroup, South Africa. We find extensive mobilization of redox-sensitive elements through oxidative weathering. Furthermore, using our data we compute a best minimum estimate for atmospheric oxygen concentrations at that time of 3 × 10(-4) times present levels. Overall, our findings suggest that there were appreciable levels of atmospheric oxygen about 3 billion years ago, more than 600 million years before the Great Oxidation Event and some 300-400 million years earlier than previous indications for Earth surface oxygenation.

Red blood cell concentrates treated with the amustaline (S-303) pathogen reduction system and stored for 35 days retain post-transfusion viability: results of a two-centre study.

BACKGROUND AND OBJECTIVES: Pathogen reduction technology using amustaline (S-303) was developed to reduce the risk of transfusion-transmitted infection and adverse effects of residual leucocytes. In this study, the viability of red blood cells (RBCs) prepared with a second-generation process and stored for 35 days was evaluated in two different blood centres. MATERIALS AND METHODS: In a single-blind, randomized, controlled, two-period crossover study (n = 42 healthy subjects), amustaline-treated (Test) or Control RBCs were prepared in random sequence and stored for 35 days. On day 35, an aliquot of 51 Cr/99m Tc radiolabeled RBCs was transfused. In a subgroup of 26 evaluable subjects, 24-h RBC post-transfusion recovery, mean life span, median life span (T50 ) and life span area under the curve (AUC) were analysed. RESULTS: The mean 24-h post-transfusion recovery of Test and Control RBCs was comparable (83·2 ± 5·2 and 84·9 ± 5·9%, respectively; P = 0·06) and consistent with the US Food and Drug Administration (FDA) criteria for acceptable RBC viability. There were differences in the T50 between Test and Control RBCs (33·5 and 39·7 days, respectively; P < 0·001), however, these were within published reference ranges of 28-35 days. The AUC (per cent surviving × days) for Test and Control RBCs was similar (22·6 and 23·1 per cent surviving cells × days, respectively; P > 0·05). Following infusion of Test RBCs, there were no clinically relevant abnormal laboratory values or adverse events. CONCLUSION: RBCs prepared using amustaline pathogen reduction meet the FDA criteria for post-transfusion recovery and are metabolically and physiologically appropriate for transfusion following 35 days of storage.

Dual Mechanism Conceptual Model for Cr Isotope Fractionation during Reduction by Zerovalent Iron under Saturated Flow Conditions.

Chromium isotope analysis is rapidly becoming a valuable complementary tool for tracking Cr(VI) treatment in groundwater. Evaluation of various treatment materials has demonstrated that the degree of isotope fractionation is a function of the reaction mechanism, where reduction of Cr(VI) to Cr(III) induces the largest fractionation. However, it has also been observed that uniform flow conditions can contribute complexity to isotope measurements. Here, laboratory batch and column experiments were conducted to assess Cr isotope fractionation during Cr(VI) reduction by zerovalent iron under both static and saturated flow conditions. Isotope measurements were accompanied by traditional aqueous geochemical measurements (pH, Eh, concentrations) and solid-phase analysis by scanning electron microscopy and X-ray absorption spectroscopy. Increasing δ(53)Cr values were associated with decreasing Cr(VI) concentrations, which indicates reduction; solid-phase analysis showed an accumulation of Cr(III) on the iron. Reactive transport modeling implemented a dual mechanism approach to simulate the fractionation observed in the experiments. The faster heterogeneous reaction pathway was associated with minimal fractionation (ε=-0.2‰), while the slower homogeneous pathway exhibited a greater degree of fractionation (ε=-0.9‰ for the batch experiment, and ε=-1.5‰ for the column experiment).

Fluctuations in Precambrian atmospheric oxygenation recorded by chromium isotopes.

Geochemical data suggest that oxygenation of the Earth's atmosphere occurred in two broad steps. The first rise in atmospheric oxygen is thought to have occurred between approximately 2.45 and 2.2 Gyr ago, leading to a significant increase in atmospheric oxygen concentrations and concomitant oxygenation of the shallow surface ocean. The second increase in atmospheric oxygen appears to have taken place in distinct stages during the late Neoproterozoic era ( approximately 800-542 Myr ago), ultimately leading to oxygenation of the deep ocean approximately 580 Myr ago, but details of the evolution of atmospheric oxygenation remain uncertain. Here we use chromium (Cr) stable isotopes from banded iron formations (BIFs) to track the presence of Cr(VI) in Precambrian oceans, providing a time-resolved picture of the oxygenation history of the Earth's atmosphere-hydrosphere system. The geochemical behaviour of Cr is highly sensitive to the redox state of the surface environment because oxidative weathering processes produce the oxidized hexavalent [Cr(VI)] form. Oxidation of reduced trivalent [Cr(III)] chromium on land is accompanied by an isotopic fractionation, leading to enrichment of the mobile hexavalent form in the heavier isotope. Our fractionated Cr isotope data indicate the accumulation of Cr(VI) in ocean surface waters approximately 2.8 to 2.6 Gyr ago and a likely transient elevation in atmospheric and surface ocean oxygenation before the first great rise of oxygen 2.45-2.2 Gyr ago (the Great Oxidation Event). In approximately 1.88-Gyr-old BIFs we find that Cr isotopes are not fractionated, indicating a decline in atmospheric oxygen. Our findings suggest that the Great Oxidation Event did not lead to a unidirectional stepwise increase in atmospheric oxygen. In the late Neoproterozoic, we observe strong positive fractionations in Cr isotopes (delta(53)Cr up to +4.9 per thousand), providing independent support for increased surface oxygenation at that time, which may have stimulated rapid evolution of macroscopic multicellular life.

Construction and evaluation of a novel humanized HER2-specific chimeric receptor.

INTRODUCTION: The human epidermal growth factor receptor 2 (HER2) represents one of the most studied tumor-associated antigens (TAAs) for cancer immunotherapy. The monoclonal antibody (mAb) trastuzumab has improved the outcomes of patients with HER2+ breast cancer. However, a large number of HER2+ tumors are not responsive to, or become resistant to, trastuzumab-based therapy, and thus more effective therapies targeting HER2 are needed. METHODS: HER2-specific T cells were generated by the transfer of genes that encode chimeric antigen receptor (CAR). Using a multistep overlap extension PCR method, we constructed a novel, humanized HER2 CAR-containing, chA21 single-chain variable fragment (scFv) region of antigen-specific mAb and T-cell intracellular signaling chains made up of CD28 and CD3ζ. An interferon γ and interleukin 2 enzyme-linked immunosorbent assay and a chromium-51 release assay were used to evaluate the antitumor immune response of CAR T cells in coculture with tumor cells. Furthermore, SKBR3 tumor-bearing nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice were treated with HER2 CAR T cells to evaluate antitumor activity. Human CD3+ T cell accumulation in tumor xenograft was detected by immunohistochemistry. RESULTS: chA21-28z CAR was successfully constructed, and both CD4+ and CD8+ T cells were transduced. The expanded HER2 CAR T cells expressed a central memory phenotype and specifically reacted against HER2+ tumor cell lines. Furthermore, the SKBR3 tumor xenograft model revealed that HER2 CAR T cells significantly inhibited tumor growth in vivo. Immunohistochemical analysis showed robust accumulation of human CD3+ T cells in regressing SKBR3 lesions. CONCLUSIONS: The results of this study show that novel chA21 scFv-based, HER2-specific CAR T cells not only recognized and killed HER2+ breast and ovarian cancer cells ex vivo but also induced regression of experimental breast cancer in vivo. Our data support further exploration of the HER2 CAR T-cell therapy for HER2-expressing cancers.

Common occurrence of a positive δ53Cr shift in Central European waters contaminated by geogenic/industrial chromium relative to source values.

Carcinogenic effects of hexavalent chromium in waters are of concern in many countries worldwide. We explored Cr isotope systematics at 11 sites in the Czech Republic and Poland. Geogenic Cr pollution was associated with serpentinite bodies at former convergent plate margins, while anthropogenic Cr pollution resulted from electroplating, tanning, and the chemical industry. Cr(VI) concentration in geogenic waters was less than 40 ppb. Anthropogenic waters contained up to 127,000 ppb Cr(VI). At both geogenic and anthropogenic sites, where known, the source of pollution had a low δ53Cr (<1‰). δ53Cr of geogenic and anthropogenic waters was up to 3.9 and 5.8‰, respectively. At both serpentinite-dominated and industrial sites, δ53Cr(VI)aq was shifted toward higher values, compared to the pollution source. At the industrial sites, this positive δ53Cr shift was related to Cr(VI) reduction, a process known to fractionate Cr isotopes. At geogenic sites, the origin of high δ53Cr(VI)aq is tentatively ascribed to preferential release of 53Cr during oxidation of soil Cr(III) and its mobilization to water. δ53Cr(VI) of industrially contaminated waters was significantly higher (p<0.001) compared to δ53Cr of waters carrying geogenic Cr(VI), implying that either the effective fractionation factor or process extent was greater for Cr(VI) reduction than for Cr(III) oxidation.

Quantifying diet-borne metal uptake in Gammarus pulex using stable isotope tracers.

Gammarids are aquatic amphipods widely used for water quality monitoring. To investigate the copper and cadmium diet-borne metal uptake in Gammarus pulex, we adapted the pulse-chase stable isotopes-based approach to determine the food ingestion rate (IR), the gut retention time (GRT) and the metal assimilation efficiencies (AE). G. pulex were fed with (65)Cu-, (106)Cd-, and (53)Cr-labeled alder leaves for 7.5h and then with unlabeled leaves for 5d. The metal stable isotope contents in the gammarids, leaves, filtered water and periodically collected feces were determined. Chromium was poorly assimilated by the gammarids; thus, Cr was used as an unassimilated tracer. The first tracer defecation occurred before the first feces harvest, indicating a gut passage time of less than 9h. A 24-h GRT and a 0.69gg(-1)d(-1) IR were estimated. The Cd AE value was estimated as 5-47%, depending on the assimilation determination method applied. The Cu AE value could not be evaluated regardless of the determination method used, most likely because of the rapid Cu regulation in gammarids in addition to analytical uncertainties when determining the Cu content in leaves. Application of the Cd AE value in the framework of the biodynamic bioaccumulation model shows that the diet-borne uptake of Cd significantly contributes (66-95%) to the metal bioaccumulation in G. pulex fed with alder leaves.

Differential isotopic fractionation during Cr(VI) reduction by an aquifer-derived bacterium under aerobic versus denitrifying conditions.

We studied Cr isotopic fractionation during Cr(VI) reduction by Pseudomonas stutzeri strain RCH2. Despite the fact that strain RCH2 reduces Cr(VI) cometabolically under both aerobic and denitrifying conditions and at similar specific rates, fractionation was markedly different under these two conditions (ε was ∼2‰ aerobically and ∼0.4‰ under denitrifying conditions).

Reactive transport modeling of chromium isotope fractionation during Cr(VI) reduction.

Chromium isotope fractionation is indicative of mass-transfer processes, such as reduction of Cr(VI) to Cr(III) during groundwater remediation. Laboratory experiments comparing batch and column treatment of Cr(VI) using organic carbon suggest that the associated isotope fractionation may be influenced by solute-transport mechanisms. These batch and column experiments were simulated using the reactive transport model MIN3P to further evaluate the effects of Cr reduction and transport on isotope fractionation under saturated flow conditions. Simulation of the batch experiment provided a good fit to the experimental data, where a fractionation factor (α53) of 0.9965 was attributed to a single, dominant Cr(VI) removal mechanism. Calibration of the column simulations to the experimental results suggested the presence of a second, more rapid Cr(VI) removal mechanism with α53 = 0.9992. Results from this study demonstrate that the interpretation of Cr isotope fractionation during reduction can be complex, particularly where multiple removal mechanisms are evident. Reactive transport modeling of Cr isotope fractionation can provide a quantitative assessment of the contaminant removal mechanisms, thus improving the application of Cr isotope measurements as a tool to track Cr(VI) migration and attenuation in groundwater.

Determination of hexavalent chromium reduction using Cr stable isotopes: isotopic fractionation factors for permeable reactive barrier materials.

Cr stable isotope measurements can provide improved estimates of the extent of Cr(VI) reduction to less toxic Cr(III). The relationship between observed (53)Cr/(52)Cr ratio shifts and the extent of reduction can be calibrated by determining the isotopic fractionation factor for relevant reactions. Permeable reactive barriers (PRB) made of Fe(0) and in situ redox manipulation (ISRM) zones effectively remediate Cr-contaminated aquifers. Here, we determine the isotopic fractionations for dominant reductants in reactive barriers and reduced sediments obtained from an ISRM zone at the US DOE's Hanford site. In all cases, significant isotopic fractionation was observed; fractionation (expressed as ε) was -3.91‰ for Fe(II)-doped goethite, -2.11‰ for FeS, -2.65‰ for green rust, -2.67‰ for FeCO(3), and -3.18‰ for ISRM zone sediments. These results provide a better calibration of the relationship between Cr isotope ratios and the extent of Cr(VI) reduction and aid in interpretation of Cr isotope data from systems with reactive barriers.

Cadmium isotopes in Bahamas platform carbonates: A base for reconstruction of past surface water bioproductivity and their link with chromium isotopes.

The distribution of cadmium (Cd) within the oceans strongly suggests that it is used as a nutrient by marine phytoplankton. Biologically induced removal of Cd from modern surface waters is accompanied by an isotopic fractionation leaving surface-waters enriched in isotopically heavy Cd. This first study focusses on tying the Cd isotopic record preserved in modern shallow platform carbonates of the Great Bahama Bank (GBB) to conditions in the upper water column, and provides a base for future studies aiming at reconstructing past bioproductivity levels in ancient ocean/basin surface waters. In addition, we compare δ114Cd values with previously published chromium (Cr) isotope values and link signals of bioproductivity with redox conditions in the surface waters. The GBB core samples yield [Cd] (21-188 µg/kg), which increases with depth alongside changes in carbonate mineralogy related to sediment supply and diagenesis. The δ114Cd values of these carbonates are mainly positively fractionated with an average of 0.11‰ ± 0.17 (2σ; n = 17) relative to the NIST 3108 reference standard. Unlike previously observed for Cr isotopes, there is no control of δ114Cd values by relative abundances of the carbonate polymorphs aragonite and calcite in the studied profile. Likewise, δ114Cd values are not correlated to major and trace element (e.g. Ca, Mg, Mn and Sr) contents. We postulate that the burial and diagenetic processes of carbonate cannot modify the Cd isotope signals. Using the experimental fractionation factor for Cd into calcite (-0.45‰), calculated seawater δ114Cd of +0.56 ± 0.17‰ is in agreement with values for modern North Atlantic Surface Seawater. This study's results suggest that δ114Cd values in carbonates are a reliable tool for reconstruction of bioproductivity levels in past surface seawaters, and open new possibilities in combination with Cr isotopes to link these with past ocean redox.

Cr stable isotopes in Snake River Plain aquifer groundwater: evidence for natural reduction of dissolved Cr(VI).

At Idaho National Laboratory, Cr(VI) concentrations in a groundwater plume once exceeded regulatory limits in some monitoring wells but have generally decreased over time. This study used Cr stable isotope measurements to determine if part of this decrease resulted from removal of Cr(VI) via reduction to insoluble Cr(III). Although waters in the study area contain dissolved oxygen, the basalt host rock contains abundant Fe(II) and may contain reducing microenvironments or aerobic microbes that reduce Cr(VI). In some contaminated locations, (53)Cr/(52)Cr ratios are close to that of the contaminant source, indicating a lack of Cr(VI) reduction. In other locations, ratios are elevated. Part of this shift may be caused by mixing with natural background Cr(VI), which is present at low concentrations but in some locations has elevated (53)Cr/(52)Cr. Some contaminated wells have (53)Cr/(52)Cr ratios greater than the maximum attainable by mixing between the inferred contaminant and the range of natural background observed in several uncontaminated wells, suggesting that Cr(VI) reduction has occurred. Definitive proof of reduction would require additional evidence. Depth profiles of (53)Cr/(52)Cr suggest that reduction occurs immediately below the water table, where basalts are likely least weathered and most reactive, and is weak or nonexistent at greater depth.

Microbially induced chromium isotope fractionation and trace elements behavior in lower Cambrian microbialites from the Jaíba Member, Bambuí Basin, Brazil.

In east-central Brazil, the Ediacaran-Cambrian Bambuí Basin has the potential to provide a record of unique geochemical responses of Earth's ocean and atmosphere evolution during this key time interval. From this perspective, we studied an interval of the upper Bambuí Basin using sedimentologic, stratigraphic, and chemostratigraphic tools. The lower Cambrian Jaíba Member of the uppermost Serra da Saudade Formation is an interval of up to 60 m-thick of carbonate rocks disposed into two shallowing upward trends. Inner to outer ramp and high-energy shoal deposits are described, in which laminated microbialites are the prevailing sedimentary facies. REE + Y data suggest contamination by iron (oxy)hydroxides that are dissociated from the riverine detritic flux. Sedimentary iron enrichment may be related to the settling of iron nanoparticles in coastal environments, diagenetic iron mobilization, or both. MREE enrichment is caused by microbial degradation of organic matter in the iron reduction zone during the anoxic early-diagenetic stage. Chromium isotopes yielded negatively fractionated values (δ53 Cr = -0.69 to -0.27‰), probably resulting from biotic and abiotic reduction of dissolved Cr(VI) to light and less toxic Cr(III) within pores of microbial mats. The δ53 Cr data of the Jaíba microbialite are thus a product of metabolic reactions in microbial mats and do not reflect seawater signal. The isotopic offset from seawater is feasible from molecular diffusion of Cr into pore water and reduction reactions occurring deep inside the mat, although the exact mechanism and consequences are not yet fully understood due to the poor preservation of metabolic reactions in the geological record. Our study suggests that Cr isotopes can be used to reconstruct Cr and other metals cycling within ancient microbial mats, and that caution should be taken when using past microbialites to infer seawater Cr records and redox state of the atmosphere and ocean.

Specific immune response genes of the guinea pig. 3. Linkage of the GA and GT immune response genes to histocompatibility genotypes in inbred guinea pigs.

The ability of guinea pigs to form immune responses specific for each of the random copolymers, L-glutamic acid and L-alanine (GA) and L-glutamic acid and L-tyrosine (GT), is under the control of distinct autosomal dominant genes. By testing for the ability to respond to these copolymers among the progeny from the reciprocal backcross mating of responder (2 x 13)F(1) animals with the appropriate nonresponder parental strain, we have demonstrated that different unigenic autosomal dominant traits control the ability to respond to GA and GT respectively. The data further shows that the GA gene is linked to the poly-L-lysine (PLL) gene and to the locus determining the major strain 2 histocompatibility specificities and that the GT gene is linked to the locus controlling the expression of major strain 13 histocompatibility specificities. Analysis of the inheritance of the GT and PLL genes among the offspring from a mating of responder (2 x 13)F(1) guinea pigs with random-bred guinea pigs unable to respond to GT or PLL demonstrate that these genes segregate away from each other. Thus, the PLL gene and the genes to which it is linked, the GA gene and the major strain 2 histocompatibility locus, behave as alleles or pseudoalleles to the GT gene and the major strain 13 histocompatibility locus.

Histocompatibility specificity.

The lymph node cells from all L-glutamic acid and L-tyrosine (GT) responder random-bred guinea pigs were susceptible to lysis by strain 2 anti-strain 13 isoantisera in the presence of complement. These same antisera were cytolytic for lymph node cells of only some of the GT nonresponder animals. However, after absorption with cells, from a nonresponder guinea pig, susceptible to lysis, the anti-strain 13 antisera were no longer able to lyse cells from any GT nonresponder guinea pigs while retaining a large measure of their cytolytic activity for cells of all GT responder guinea pigs. Thus, at least two major strain 13 histocompatibility specificities are expressed on the cells of random-bred guinea pigs. The genetic locus controlling the expression of only one of those strain 13 histocompatibility specificities is linked to the GT immune response gene.

Cell-mediated lympholysis. Importance of serologically defined H-2 regions.

The cell-mediated lympholytic capability of mouse spleen cells stimulated in mixed lymphocyte culture is related to the major histocompatibility complex genotype on target lymphocytes. The strain combinations AQR-B10. T(6R) and B10.A(4R)-B10.A(2R) that result in significant mixed lymphocyte culture activation do not mediate cell-mediated lympholysis on sensitizing target lymphocytes; serologically defined regions (H-2K and H-2D) are identical within each combination. H-2K or H-2D region disparity alone does not cause cell-mediated lympholysis. However after mixed lymphocyte culture activation as seen with B10.A-B10.T(6R), a target cell bearing only an H-2K region difference from the effector cell is sensitive to cell-mediated lympholysis. Likewise an H-2D region difference is an adequate target after mixed lymphocyte culture activation of the effector cell in the combination B10.A(2R)-B10.D2.

Escape from isoantiserum inhibition of lymphocyte-mediated cytotoxicity.

Isoantiserum (IS) inhibition of lymphocyte-mediated cytotoxicity (LMC) was studied using an in vitro (51)Cr release assay system. In the early phase of incubation, LMC was competitively inhibited by IS. However, as the incubation continued, LMC irreversibly overcame IS inhibition (the "escape" phenomenon). Addition of fresh antiserum did not alter the course of the escape. Low-temperature incubation of isoantibody-coated target cells delayed the onset of the escape. We have excluded the possibility that the escape phenomenon is induced by complement or by LMC mediated by antigen-antibody complex. It is hypothesized that antibody directed toward an actively metabolizing target cell induces an alteration in the cell membrane that alters further interaction with the antibody. However, sensitivity to lymphocyte cytotoxicity is maintained.

Distribution of labeled lymph node cells in mice during the lymphocytosis induced by Bordetella pertussis.

The mechanism by which Bordetella pertussis organisms and their products induce lymphocytosis in mice was analyzed in terms of the localization of syngeneic Cr-51-labeled lymph node cells. Labeled lymphoid cells incubated in vitro with the supernatant of B. pertussis cultures and then injected intravenously into normal recipients, or labeled cells injected into pertussis-treated recipients were unable to "home" to lymphoid organs but persisted for long periods in the blood. In animals "equipped" with a population of Cr-51-labeled lymphoid cells, administration of B. pertussis organisms or culture supernatant effected a shift of radioactivity from lymph nodes and spleen into the peripheral blood, coincident with the lymphocytosis. In in vitro experiments it was found that the active principle could bind to both erythrocytes and lymphocytes and could spontaneously elute from these cells onto labeled lymphocytes which were then unable to home efficiently. The data suggest that Bordetella pertussis-induced lymphocytosis involves a reversible attachment of the pertussis factor onto the surfaces of lymphocytes which prevents their recirculation to lymphoid organs. Recirculating lymphocytes are presumably affected as they emerge from lymphoid organs to enter the blood.