Screening microorganisms for the biotransformation of the 3-keto-delta(4,9(10))-19-norsteroid RU27987 (Trimegestone) resulted in the isolation of nine identified metabolites, some of them being selectively produced by different strains. Eight metabolites were found to be hydroxylated on various positions of the rings, and one was additionally epoxidized. These microbial metabolites could be used as chromatographic standards and two of them were found identical to the unknown major human metabolites. Moreover, most microbial metabolites were produced in sufficient amounts to be tested for their biological activities. All these features demonstrate the usefulness and versatility of microbial biotransformation systems as a tool for early identification and convenient production of potentially active mammalian and non-mammalian metabolites.