Streptomyces ravidus S50905 was found to produce a new antibiotic, deacetylravidomycin N-oxide, together with ravidomycin and deacetylravidomycin in a culture medium containing sodium anthraquinone-beta-sulfonate. The structure of this new compound was determined from NMR and mass spectrometric data, and further confirmed by chemical synthesis from deacetylravidomycin. Deacetylravidomycin N-oxide was antitumor active against P388 leukemia and Meth A fibrosarcoma in a wide range of doses, and considerably less toxic than deacetylravidomycin. Its antibacterial activity was less potent than deacetylravidomycin. Ravidomycin N-oxide was also synthesized from ravidomycin and its biological properties were tested.