We have isolated a new antitumor antibiotic, senacarcin A, from fermentation liquors of Streptomyces endus subsp. aureus (NRRL 12174). This communication reports its production, isolation, physical properties, structural inference (similar UV spectrum to griseolutein A but different molecular formula, inferred to have a phenazine skeleton with unique side chains), antimicrobial activity (mainly against Gram-positive bacteria), and antitumor activity (inhibitory effect against sarcoma 180 in ddY mice; T/C 0.33 when 9.38 mg/kg/day was injected intraperitoneally once daily for 6 days). Senacarcin A is a new antitumor antibiotic.