Several kinds of anthracyclines having γ-rhodomycinone as the aglycone were isolated from Streptomyces cosmosus TMF 518, and their derivatives were prepared by chemical modification. We tested their differentiation inducing activity in Friend leukemia cells and clarified their structure activity relationship as follows: 1) The aglycone, γ-rhodomycinone, had no differentiation inducing activity but was cytotoxic; 2) the compounds with two sugar chains at both C7 and CIO had more potent differentiation inducing activity than those with only a sugar chain at C-10; 3) cosmomycin C was the most favorable candidate for an anticancer agent of all anthracyclines tested, because the value of ED50 (cytotoxicity)/ED50 (differentiation) was as high as 3000; and 4) the increase in differentiation inducing activity and cytotoxicity was not always in parallel.