It has been shown that the antitumor antibiotics daunomycin (1) and adriamycin (4) are metabolized by microorganisms in a fashion similar to their metabolism by mammalian cells. Both the fungus Mucor spinosus and its cell-free extracts reduce the 13-keto group of daunomycin to give daunomycinol (2) by a TPNH-dependent process. Cell-free extracts of Streptomyces steffisburgensis convert adriamycin and daunomycinol to their 7-deoxyaglycones (5) and (3) by DPNH-linked reductive glycosidic cleavage. Cell-free extracts of the latter organism convert 7-deoxyadriamycinone (5) to 7-deoxyadriamycinol aglycone (6) by TPNH-linked 13-keto reduction.