Warfarin, an anticoagulant and "metabolic probe" for cytochrome P-450 isozyme multiplicity, was metabolized by the fungus Cunninghamella elegans (ATCC 36112) to yield the previously unreported metabolite 3'-hydroxywarfarin. This metabolite was isolated from cell suspension cultures and characterized by analytical (HPLC) and spectral (EI-MS, PMR) comparisons with synthetic 3'-hydroxywarfarin.