Abstract Testosterone (T) and dihydrotestosterone (DHT) were found to bind to a specific protein in genital skin fibroblasts of a patient with 5α-reductase deficiency. While maximum number of binding sites ( B max) were similar for both androgens, the apparent dissociation constant ( K D ) of the androgen receptor for T was somewhat greater than for DHT. In competition studies of [ 3H]-T bound to the receptor with unlabeled T or DHT, the inhibitor constant ( K i ) for T was two to three fold greater than the K i for DHT. Also, the dissociation rate constant ( k D ) for [ 3H]-T bound to the receptor was greater than for [ 3H]-DHT ( t 1 2 for T = 10 h and t 1 2 for DHT = 74.5 h). These results suggest that T may play a role in the sexual differentiation of male patients with 5α-reductase deficiency during their fetal life and at puberty. The low degree of masculinization during fetal life would be explained by the lower affinity and faster turnover rate of the T-reeeptor complex relative to the DHT-receptor complex. The relatively greater masculinization observed at puberty would be explained by the higher plasma T levels attained in the adult when compared to the fetus.