Testicular biopsy specimens from oligozoospermic infertile patients are characterized by different types of spermatogenic impairment in adjacent seminiferous tubules, a phenomenon called mixed atrophy. In order to evaluate possible involvement of the state of Sertoli cell differentiation, the distribution pattern of anti-Müllerian hormone (AMH), vimentin and cytokeratin intermediate filament proteins was investigated by means of immunohistochemistry. AMH immunoactivity occurs in Sertoli cells of the normal postnatal prepubertal testis, but it is absent in the adult testis with normal spermatogenesis. In the case of mixed atrophy, AMH immunoactivity was found in Sertoli cells of tubules showing spermatogenic arrest at the level of spermatogonia and in tubules showing Sertoli-cell-only (SCO) syndrome. Vimentin was expressed regularly in Sertoli cells independent of spermatogenic impairment or the state of Sertoli cell differentiation. Cytokeratin immunoactivity occurs in Sertoli cells of the normal postnatal prepubertal testis. Furthermore, cytokeratin expression was found in Sertoli cells of tubules showing spermatogenic arrest at the level of spermatogonia and in some SCO tubules. Co-expression of AMH and cytokeratin suggests that spermatogenic impairment such as spermatogenic arrest and SCO syndrome in human seminiferous tubules is associated with a population of Sertoli cells showing a prepubertal stage of development. The different pattern of AMH and cytokeratin expression in SCO tubules indicates that Sertoli cells in SCO syndrome show a mosaic pattern of differentiation.