It has been established, that Leydig cells of the human testis possess neuroendocrine properties and are therefore a member of the diffuse neuroendocrine (paraneuron) system. In the present study, we examined whether Leydig cells of adult (51-86 year of age) and developing (between the 15th and 36th week of gestation) human testes are immunopositive for glial cell-specific antigens such as glial fibrillary acidic protein (GFAP), galactocerebroside (GalC), cyclic 2',3'-nucleotide-3'-phosphodiesterase (CNPase), A2B5-antigen (A2B5) and O4-antigen (O4). With the use of Western blots and dot blot analyses, respectively, GFAP, CNPase, GalC, A2B5 and O4 were found in whole testes and Leydig cell protein extracts of adult men. Corresponding immunohistochemical studies revealed presence of these antigens in the cytoplasm of Leydig cells both of adult testes and testes during prenatal development. Some differences in staining intensity of single antigens were observed probably depending on the functional and/or developmental stage of the single cells. In addition, GFAP-, GalC- and CNPase-immunopositivity was found in numerous Sertoli cells of the seminiferous tubules. Moreover, some connective tissue cells (compartmentalizing cells or Co-cells) of the intertubular space showed immunopositivity for CNPase, A2B5 and GalC. The results obtained show that Leydig cells of the human testis, in addition to their endocrine, neuronal and neuroendocrine features, possess qualities of both astrocytes and oligodendrocytes and thus show qualities of multipotential cells. Leydig cells probably differentiate to a phenotype that is characteristic for cells in the developing nervous system. Furthermore, the established immunohistochemical similarities are consistent with the assumption that foetal and postnatal Leydig cells are of common origin.