Azinomycins A and B, isolated from the culture broth of Streptomyces griseofuscus S 42227, were examined for antitumor activities against P388 leukemia, P815 mastocytoma, B-16 melanoma, Ehrlich carcinoma, Lewis lung carcinoma and Meth A fibrosarcoma. Azinomycin B was markedly effective against the intraperitoneally inoculated tumors such as P388 leukemia, B-16 melanoma and Ehrlich carcinoma. The intraperitoneal administration of azinomycin B showed 57% survivors for 45 days and 193% ILS against P388 leukemia. For Ehrlich carcinoma, azinomycin B gave 161% ILS and 63% survivors for 45 days, but solid tumors such as Lewis lung carcinoma and Meth A fibrosarcoma were not susceptible to repeated injection of this substance. Azinomycin A was somewhat less effective than azinomycin B for the tumor systems tested.