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Thymus development and function are under the influence of hormones secreted by the gonads and pituitary. On the other hand, thymurus is crucial for the development of reproductive capacitities in female and male rats and we have shown that a factor derived from the prepubertal rat thy mus has antigonadotropic effect in ovarian and testis cells in vitro. In the present paper we show that the rat thymic factor which modulates gonadotropin action in the gonads is an heparin-binding factor. This capacity was also used as a useful tool to obtain this activity from semipure extracts. An acetone extract was prepared from 15 day old male rats and subjected to molecular filtration chromatography. The activity, of those fractions was investigated in a testis cells biossay, by measuring testosterone secretion under basal and hCG-stimulation. Active fraction were processed in a heparin-Sepharose affinity column. We found that fractions that eluted with 0.6 and 2M NaCl/10mM Tris had biological specific activity. The electrophoretic procedure showed that the apparent molecular weight of the Heparin Sephadex binding factor is 60 kDa. Since this factor was obtained from a protein peak that eluted in the volume of carbonic anhidrase a dimerization process could be involved. Present results show that the rat thymus has an heparin-binding factor that interacts with hCG in testis cells. This factor could play an interesting role in the mutual influence between thymus and gonads. (AU)