An aclacinomycin-negative mutant strain KE303 which required aklavinone aglycone for the production of anthracycline antibiotics was derived from Streptomyces galilaeus, and employed for the glycosidation of various anthracyclinones. epsilon- gamma- and beta-Rhodomycinones, epsilon-isorhodomycinone, epsilon- and beta-pyrromycinones and chemically modified aklavinones were found to be glycosidated to the biologically active anthracyclines, when they were fed to the growing culture. However, the feeding of daunomycinone, 13-deoxydaunomycinone, adriamycinone and steffimycinone did not yield any glycoside. The bioconversion of presumptive precursor glycosides revealed that aclacinomycin A is biosynthesized by the step-wise glycosidation from aklavinone vai aklavin and MA144 S1.