Novel substituted 1,8_diazaanthraquinone structures of diazaquinomycins A and B were determined by the application of nmr spectroscopy. Diazaquinomycins A') (1) and B (z), produced by Streptomyces sp. OM-704, exhibit antibacterial activities against Gram-positive bacteria and are antimetabolites of folate metabolism in Streptococcus faecium IF0 3181. This inhibitory activity is reversed by thymidine, leucovorin, and dihydrofolic acid.