The aims of this study were to provide an overall comparison of the in-vitro protein secretory profile of seminiferous tubules (ST) isolated from man and the rat, and to identify specific proteins secreted by both species. Two-dimensional SDS PAGE was used to compare the profile of proteins secreted by cultured ST from 15 men undergoing orchidectomy with those secreted by ST isolated at stages VI-VIII of the spermatogenic cycle from normal adult rats, and by ST at the same stages isolated from rats pretreated with methoxyacetic acid (MAA) in order to deplete both pachytene spermatocytes and round spermatids. Two abundant groups of proteins not present in the medium of rat ST were secreted consistently by human ST, though the profile of proteins secreted by human ST was otherwise more variable than in the rat. Twelve proteins secreted by ST isolated from humans were identified as possible homologues of rat ST-secreted proteins, including the major rat Sertoli cell products SGP-1 and SGP-2, and an androgen-regulated protein which derives in the rat from round spermatids. Otherwise, the majority of the 12 potential homologues corresponded to proteins which in the rat are secreted by ST from both normal and germ cell-depleted testes or by ST from germ cell-depleted testes only, suggesting that they are probably secreted by Sertoli cells and/or peritubular cells. Based on the potential homology of the proteins identified, our results suggest, first, that these proteins may play an important role in spermatogenesis and second, that the profile of proteins secreted by human ST is more akin to that secreted by ST isolated from germ cell-depleted rats.