This study concerns the nature of two different cell populations in a juvenile granulosa cell tumor (GCT) of the infantile testis. Immunohistochemical features of the tumor were compared with those of normal infantile testes (six cases) and ovaries (six cases). The testicular neoplasm showed follicles, cysts and solid nodules composed of an internal layer of polyhedral cells that expressed cytokeratin and vimentin. Most of the follicles and nodules were surrounded by an external layer of spindle cells that reacted to muscle-specific actin, vimentin, and focally to desmin. A neoplastic rather than reactive origin of the spindle cell population is favored by their concentric arrangement in a peritubular-like or theca-like fashion and by their immunohistochemical correlation with normal peritubular-myoid and theca external cells. Sertoli and granulosa cells of normal infantile gonads were positive for cytokeratin and vimentin; peritubular myoid and theca externa cells expressed muscle-specific actin, vimentin, and focally desmin. The occurrence of two well-differentiated components in the tumor favors its origin from the primitive specialized gonadal stromal cell that during neoplastic transformation develops bidirectional differentiation toward epithelial-like and smooth muscle-like lineages. The possibility that this tumor is composed of immature Sertoli and peritubular myoid cells is discussed.