Abstract: Aclacinomycin A (aclarubicin; ACM-A), an anthracycline antibiotic, is produced by the culture of Streptomyces galileus isolated from a soil specimen collected in Shanghai.Experiments demonstrated that ACM-A exhibited a significant cytotoxicity against murine leukemia P₃₈₈ and human stomach carcinoma cells (SGC-7901) in vitro. ACM-A was shown to suppress the clonal growth of P₃₈₈ stem cells with an ID₅₀ of approximately 0.0085 μg/mh In transplantable tumor system, ACM-A 1.25～3.0 mg/kg ip prolonged the life-span of mice bearing P₃₈₈ leukemia or Ehrlich carcinoma, and cured mice bearing P₃₈₈ leukemia by 56.3%. ACM-A also inhibited the growth of solid sarcoma-180 in mice. ACM-A exerted the same degree of inhibitory action on ³H-TdR incorporation into DNA as adriamycin and less than daunomycin in cultured P₃₈₈ cells. It also inhibited ³H-uridine and ³H-leucine incorporation into RNA and protein, respectively. The ip acute LD₅₀ of ACM-A was found to be 32.1 mg/kg in BDF₁ mice.