A new antitumor antibiotic, decilorubicin has been found in the culture broth of Streptomyces virginiae MF266-g4 (FERM P-5401, ATCC 31910) which was isolated from a soil sample collected at Nishi-Shinagawa, Tokyo. The antibiotic exhibits an antibacterial activity against Gram-positive bacteria and a prolongation effect on experimental mouse leukemia L-1210. Decilorubicin consists of an anthracyclinone chromophore, rhodosamine and four other sugars. Decilorubicin dihydrochloride inhibited the growth of Gram-positive bacteria. A prolongation effect in the survival period of mice implanted with the mouse leukemia L-1210 cells was observed after intraperitoneal injection of decilorubicin dihydrochloride. Acute LD of decilorubicindihydrochloride in mice was 50 - 100 mg/kg by the intravenous or intraperitoneal injection.