Semisynthetic derivatives of daunomycinone with 7,9-isopropylacetal, 7-O-methyl, 7-O-(4-penten-2-yl), and 7-O-(2-hydroxyethyl) substituents were converted by Streptomyces peucetius var. caesius (an adriamycin-blocked mutant) into 7-deoxy-13-dihydrodaunomycinone, while daunomycinone was transformed into 13-dihydrodaunomycinone (predominantly) and 7-deoxy-13-dihydrodaunomycinone. S. coeruleorubidus mutants 24-74 (accumulating aclavinone derivatives instead of daunomycin and related compounds) and 96-85 (producing no anthracycline substances), and S. aureofaciens B-96 (a tetracycline-blocked mutant) transformed the above substrates into the corresponding 13-dihydro derivatives, with the exception of 7,9-isopropylacetal daunomycinone which remained intact. 7-O-Propyn-1-yl daunomycinone was not transformed by any of the strains used under the conditions.