Testosterone and dihydrotestosterone (DHT) were estimated by radioimmunoassay in human seminal plasma. Testosterone concentrations showed no significant differences between fertile and infertile semen samples, whereas DHT concentrations were significantly lower in azoospermic and oligozoospermic samples. It is concluded that testosterone derives essentially from the accessory sex glands, whereas DHT is mainly of testicular or epididymal origin. The low DHT concentrations found in seminal plasma of oligozoospermic and azoospermic patients is probably due to defective epididymal conversion of testosterone to DHT.