Sporamycin is an antitumor antibiotic isolated from the culture filtrate of Streptosporangium strain No. PO-357. Hydrolysate of the antibiotic showed at least 12 kinds of amino acid, and the molecular weight was calculated approximately as 8,500 approximately 9,000. Antitumor activities of Sporamycin were examined on murine tumors according to different schedules of treatment. After nine daily doses of the antibiotic had been given to dd mice bearing Ehrlich ascites carcinoma, significant increase of lifespan was observed at doses of 5.0 approximately 1.3 mg/kg. A marked decrease in tumor size of sarcoma-180 transplanted subcutaneously was observed when the mice were treated by daily or single injection of Sporamycin. Against leukemia P-388 and L-1210 inoculated intraperitoneally into CDF1 mice, the antibiotic increased lifespan of leukemic mice. The antibiotic also inhibited significantly the growth of human tumor cells in vitro.