1. Biotransformation of warfarin by the fungus Beauveria bassiana (ATCC 7159) yielded the first reported phase II warfarin metabolite, 3'4'-dihydroxywarfarin-3'-[4-methoxyglucoside], another previously unreported metabolite, 3',4'-dihydroxywarfarin and also 4'-hydroxywarfarin. 2. Biotransformation of warfarin by Streptomyces rimosus (NRRL 2234) yielded the previously unreported metabolites, 12-hydroxywarfarin, 4'-hydroxy-11-methoxywarfarin, and also 7-hydroxywarfarin and 4'-hydroxywarfarin, which have not been previously reported as biotransformation products from this organism. 3. Hplc-nmr has been used to identify biotransformation products of warfarin by S. rimosus directly from the microbial broth without prior isolation and purification.