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. In this paper, we wish to report a novel 1,8_diazaanthraquinone structure (1) for diazaquinomycin A by the application of nmr spectroscopy. The antibiotic (l), deep red needles, mp 291-295"C, EI Mass: Mt ~/z_ 354.157 (Calcd. for C20H22N204, 354.157), showed a characteristic UV absorption, heath nm (E) 250 (11800, sh), 260 (13600, sh), 278 (20100, sh), 286 (21700), 309 (9760), 321 (8950), 367 (4130), and 490 (1150), and IR absorption, ","iL 1670, 1625 cm-l (carbonyl) for an anthraquinone like structure.2) The 13C-nmr spectrum (100 MHz, CDCl 3 + CD30D, Table 1) showed eleven carbon signals, 6c 182.9 and 173.9 (quinone carbonyls), 6c 162.2 (two amide carbonyls), AC 151.2, 136.8, 135.0, and 117.3 (each two substituted aromatic carbons), 6c 32.4 and 22.9 (each two methylene carbons), and 6c 14.6 and 12.9 (each two methyl carbons), indicating the existence of a symmetric structural unit in the molecule.