This report describes the simultaneous radioimmunoassay of plasma testosterone (T) and 5a-dihydrotestosterone (DHT), using as binding reagent a highly specific antiserum which reacted significantly with T and DHT. Utilizing a one step celite microcolumn to separate T from DHT, we have measured these two androgens accurately in small aliquots of plasma from both male and female subjects, Plasma concentrations of T and DHT expressed as mean ± S.D. in ng/ml were respectively:1) in male subjects:a. normal adult: 4.9 ± 1.6 (T) and 1.3 ± 0.8 (DHT); b. hypogonadal: 0.24 ± 0.08 (T) and 0.20 ± 0.10 (DHT); c. prepubertal: 0.14 ± 0.02 (T) and 0.05 ± 0.02 (DHT).2) in female subjects:a. normally menstruating, irrespective of time of cycle: 0.22 ± 0.07 (T) and 0.34 ± 0.17 (DHT); b. postmenopausal: 0.29 ± 0.08 (T) and 0.15 ± 0.07 (DHT); c. ovariectomized: 0.24 ± 0.06 (T) and 0.09 ± 0.01 (DHT). When plasma levels of T and DHT were measured during a complete menstrual cycle, the levels of these steroids were found to be higher during the luteal than during the follicular phose. The ratio of plasma T:DHT concentrations was five times higher in normal adult male (3.1) than in menstruating female subjects (0.6).