The enzyme estradiol 17 beta-dehydrogenase (17 beta-ED) [E.C.1.1.1.62] from human placenta was purified to homogeneity by the initial steps of a published procedure, followed by an affinity chromatography step in Reactive Blue 2-Sepharose, eluting with NADP. The pure enzyme is not specific for estrogenic substrates, it also catalyzes the oxidation-reduction of several androgens and progesterone (i.e. dehydroepiandrosterone, androstenedione, 5 alpha-dihydrotestosterone, and 20 alpha-dihydroprogesterone). The comparison of the kinetic parameters for these substrates, shows that dehydroepiandrosterone could be a physiological ligand of the enzyme, and consequently involved in the control of its function in estrogen metabolism.