Antibodies have been raised in rabbits against 3α,17β-dihydroxy-5α-androstane-6-0-carboxymethyloxime coupled with Cohn's fraction IV-4. The antiserum exhibited significant cross reactions with 5β-androstane-3α,17β-diol, 5α-dihydrotestosterone, and testosterone. No cross reactions were observed with 5α-androstane-3β,17β-diol and 5-androstene-3β,17β-diol. The methodological criteria for the measurement of 5α-androstane-3α,17β-diol in human plasma were as follows: The specificity was ensured by separating the cross reacting steroids by thin layer chromatography. The intraassay and interassay coefficients of variation were found to be 6.2 and 10.2 %, respectively. The sensitivity was 30 pg. The recovery of different amounts of 5α-androstane-3α,17β-diol added to human plasma (80, 120, and 200 pg) yielded 91.3, 92.5, and 93.5%, respectively. The following concentrations of 5α-androstane-3α,17β-diol have been determined in human plasma (mean ± sd, ng/dl): Normal males: 18.98 ± 5.9; normal females: 2.65 ± 0.27; females with idiopathic hirsutism: 11.9 ± 6.4; pre-pubertal children: not detectable.