Cytochrome P-450 specific for steroid 17α-hydroxylation (P-450_17α) was immunohistochemically observed in pig adrenal cortex, testis and ovary by the biotin-streptavidin method using a specific antibody against P-450_17α purified from neonatal pig testis. In the adrenal cortex, P-450_17α was present in the zona fasciculata and reticularis while no immunoreactivity was observed in the zona glomerulosa, confirming the absence of 17-hydroxylated steroid synthesis in the zona glomerulosa. In the testis, P-450_17α was present exclusively in Leydig cells and immunoreactivity was absent in seminiferous tubules. In the ovary, immunoreactivity was observed only in the theca interna but not in the membrana granulosa of follicles. Among the tissues examined, the relative intensity of immunoreactivity was greatest in the Leydig cells, and progressively less in theca interna cells, outer fasciculata cells and inner fasciculata and reticularis cells.