Friend erythroleukemia cells can be induced in vitro to express various normal functions such as hemoglobin biosynthesis and lose tumorigenicity. Many chemicals such as dimethylsulfoxide (DMSO) and hexamethylene bisacetamide (HMBA) are known as inducers. In searching for new antitumor antibiotics, we used the differentiation of Friend leukemia cells as an assay system, and found that actinomycin V, cosmomycins, and (+)-trichostatic acid induce Friend cell differentiation strongly. We found a more potent compound in the culture liquid of Streptomyces sioyaensis AJ 9451, in which (+)-trichostatic acid was also found. This paper describes the isolation of one of the active components, FL-657A, from the culture broth of St. sioyaensis and its identification with (+)-trichostatin A. FL-657A induced well at extremely low concentrations (0.5 nM to 50 nM), and 90% of the F5-5 cells treated with 9 nM FL-657A biosynthesized hemoglobin within 6 days. At concentrations above 4.5 nM of FL-657A the growth of F5-5 cells was inhibited. FL-657A induced the differentiation of not only F5-5 but also of other types of Friend leukemia cells, such as C9-6, insensitive to DMSO, which are derived from the DDD mouse, and C1745A, TS19-101S, and TR19-9R, which are derived from the DBA/2 mouse. A hydroxamic acid residue of (+)-trichostatin A seems to give great influence on the Friend cell differentiation. Trichostatin C, with an O-/?-glucopyranosyl hydroxamate group, was reported to have about 200 times lower inducing activity than that of (+)-trichostatin A.