Sporamycin is an antitumor antibiotic isolated from the culture filtrate of Streptosporangium strain no. PO-357. Hydrolysates of the antibiotic showed at least 12 amino acids, and the MW was calculated approximately as 8500-9000. Antitumor activities of Sporamycin were examined in murine tumors according to different schedules of treatment. After 9 daily doses of the antibiotic were given to dd mice bearing Ehrlich ascites carcinoma, significant increase of lifespan was observed at doses of 5.0-1.3 mg/kg. A marked decrease in tumor size of sarcoma 180 transplanted s.c. was observed when mice were treated by daily or single injection of Sporamycin. Against leukemia P-388 and L-1210 inoculated i.p. into CDF1 mice, the antibiotic increased the lifespan of leukemic mice. The antibiotic also inhibited the growth of human tumor cells in vitro.