Human placental 3 beta-hydroxysteroid dehydrogenase/5----4-ene isomerase (<< 3 beta-HSD >>) purified from human placenta transforms C-21 (pregnenolone and 17 alpha-hydroxy pregnenolone) as well as C-19 (dehydroepiandrosterone and [[ androst-5-ene-3 beta, 17 beta-diol ]]) steroids into the corresponding 3-keto-4-ene-steroids and is thus involved in the biosynthesis of all classes of hormonal steroids.