A rapid and relatively simple, but specific, radioimmunoassay for the potent androgen, androstanediol (3 alpha-diol), is described. Despite the availability of a nonspecific C-19 androgen antibody requiring a 17 beta-hydroxy group, androstanediol can be measured in plasma by prior purification of a plasma solvent extract using a Celite microcolumn. Values obtained do not differ from those previously reported using more complicated chromatographic techniques.