We studied the tissue distribution and cellular localization of 5 alpha-reductase 2, the human prostatic isoenzyme, in different human tissues, both cryostat-sectioned and paraffin-embedded. Polyclonal antibodies raised in rabbits against a native peptide (C-terminal amino acids 229-254) or synthetic peptides (amino acids 234-245), either as carrier-conjugated linear peptides or multiple antigen peptides (MAP), were assayed for specificity and sensitivity with Western blotting and an ELISA system. One antibody showing monospecificity on Western blots and in ELISA was used for immunohistochemical detection of the respective antigen in tissues from male and female subjects. Positive cells were found (with decreasing intensity) in inner epithelial sheath of hair follicles, pyramidal cells of the cerebral cortex, hepatocytes and bile duct cells, prostate epithelial cells, seminal vesicle epithelial cells, endothelial cells of small vessels, fat cells, fibrocytes of genital and extragenital organs, and smooth muscle cells of prostate and seminal vesicles. Some variation in the immunoreactivity of testis and ovary tissue was seen with different antibodies. 5 alpha-Reductase 2 is obviously not restricted to androgen target organs in the male, but is present in a large number of cells and tissues in both males and females.