The antitumor activity of a new antibiotic, stubomycin, against various murine tumors was studied. Stubomycin showed antitumor activity against Sarcoma 180, IMC-carcinoma and Meth-A tumor. In particular, all of the mice inoculated i.p. with IMC-carcinoma were cured when they were given nine successive injections of stubomycin. Stubomycin also showed remarkable antitumor effects against Sarcoma 180 and Ehrlich ascites carcinoma when administered concomitantly with bleomycin.