Unconjugated androstenediol has been identified and measured in the plasma of normal adults. Identification of the steroid in ether extracts of plasma was based upon its affinity for “testosterone binding globulin,” chromatographic mobility before and after derivative formation, and identity after acetylation with carrier androstenediol diacetate upon recrystallization. By means of competitive protein binding analysis, peripheral plasma androstenediol levels were determined to be 68.2 ± 24 (sd) ng/100 ml in normal women, 124 ± 45 ng/100 ml in normal men. The androstenediol concentration in men averages twenty percent of the testosterone level, that of women is nearly twice their testosterone level. Since available data indicates that androstenediol is moderately androgenic, this steroid appears to contribute significantly to total androgenicity in women and may play a role in certain hyperandrogenic states.