In the course of our screening program for new antitumor antibiotics, a Streptomycete identified as Streptomyces griseofuscus S42227 was found to produce two new compounds possessing good antitumor activity in mice, which we named azinomycins A(1) and B(2), and three biologically inactive compounds structurally related to 1 and 2 (5, 6 and 7). The structures of azinomycins A and B, new antitumor antibiotics produced by a strain of Streptomyces, were determined on the basis of their spectral and chemical properties. The structures of three related metabolites coproduced with these antibiotics were also determined.