Four monohydroxy derivatives of androst-4-ene-3,17-dione (4AD) and two of androst-4-ene-3,6,17-trione (4AT), derived from microbial transformation of 4AD and 4AT, were purified and identified by UV, TR, mass, C-13-NMR, and H-1-NMR spectroanalyses. These derivatives were evaluated as inhibitors of human placental aromatase. They showed dose-dependent inactivation of aromatase in the presence of reduced nicotinamide adenine dinucleotide phosphate. Among them, 6 alpha-hydroxyandrost-4-ene-3,17-dione (6 alpha-OH-4AD) was the most potent, exhibiting 97.8% inhibition at 100 mu M; this value was 1.04-fold higher than the inhibition shown by 4AT (93.6%). The other derivatives, 16 beta-OH-4AT, 15 alpha-OH-4AD, 15 beta-OH-4AD, 6 beta-OH-4AD and 12 alpha-OH-4AT, inhibited the aromatase 80.4%, 63.6%, 61.8%, 30.1%, and 9.6%, respectively, at the same concentration.