Anti-5 alpha-reductase autoantibodies in the serum of patients with prostatic cancer.
 Human sera were tested for their ability to inhibit 5 alpha-reductase binding of a potent inhibitor of the enzyme.
 Thirty one of 227 serum samples from patients diagnosed or suspected of prostatic cancer had a significant inhibitory activity, whereas 128 serum samples from other patients were inactive.
 The majority of the inhibitory activity was in the IgG fraction purified by chromatography on a protein A-Sepharose affinity column and an anti-human IgG-agarose column.
 IgG fractions from non-inhibitory sera were inactive.
 Inhibitory IgG also inhibited the enzymatic activity of microsomal 5 alpha-reductase from liver, ventral prostate and preputial gland of rat, and liver, prostate, and facial skin of human.
 The inhibitory IgG had no effect on NADH-menadione reductase or 17 beta-hydroxysteroid dehydrogenase.
 These results suggest that 5 alpha-reductase autoantibodies are present in the blood of some prostatic cancer patients.
