An enzyme immunoassay system has been prepared using an anti-class III alcohol dehydrogenase (ADH III) monospecific antiserum. The contents of immunoreactive protein in two autopsied specimens of human testicles determined by the test system were 1.288 and 1.047 mg/g wet weight. ADH III was immunohistochemically located in all cells of human testicles; especially large amounts of the enzyme were found in secondary spermatocytes and interstitial endocrine cells.