A radioimmunoassay for 3α-androstanediol (3α-diol) in human peripheral plasma is described. The antigen was prepared by reacting androsterone with succinic anhydride forming androsterone-3α-succinate and then reducing the 17 ketone to 3α-diol with sodium borohydride. This was then conjugated with bovine thyroglobulin using the mixed anhydride reaction. Antibody generated in rabbits had high affinity for 3αdiol and dihydrotestosterone and a lower order of cross reaction with testosterone and 3(β-diol. Plasma with 3H indicator is extracted with solvent and purified on a paper chromatogram (Bush-A system). Separation of bound from free steroid affected by ammonium sulfate precipitation. Studies indicate good precision and accuracy. Non specific blank values are 8 ± 8 (sd) pg per sample and sensitivity is about 20 pg. Male values are 13.7 ± 4 (sd) ng and female values are only 2.0 ± 0.6 ng per 100 ml with a significant sex difference. Male and female prepubertal values are very low, < 2 ng per 100 ml. In a few pathological states, 3αdiol levels generally paralleled testosterone values.