The soluble enzyme, estradiol-17 beta dehydrogenase from human term placenta, appears to co-purify with a second soluble enzyme, 20 alpha-hydroxysteroid dehydrogenase. The enzyme, which had been partially purified by affinity chromatography, fractionated on a preparative electrophoresis gel to a homogeneous preparation containing both estradiol-17 beta dehydrogenase and 20 alpha-hydroxysteroid dehydrogenase activities in a ratio of approximately 100:1. Analytical polyacrylamide disc-gels resolved this homogeneous preparation as a single band by both protein and activity staining techniques. Homogeneous enzyme inactivated and affinity-radioalkylated by 16 alpha-[2'-14C]bromoacetoxyprogesterone or 16 alpha-[2'-14C] bromoacetoxyestradiol 3-methyl ether, and when analyzed by SDS disc-gel electrophoresis, gave a single protein band which corresponded identically to the radioactivity peaks. These observations support the hypothesis that estradiol-17 beta dehydrogenase and 20 alpha-hydroxysteroid dehydrogenase represent dual oxidoreductase activity in one enzyme. Preparative disc-gel electrophoresis, a technique which has not been previously adapted to purification of these human placental enzyme activities, was useful to rapidly (3 days) effect a 15-fold enrichment of the estradiol-17 beta dehydrogenase specific activity from "heat-treated cytosol". Thus, laboratory-scale preparative disc-gel electrophoresis is useful for rapid, small-scale enrichment of this soluble enzyme.