Although several antitumor diazo ketones have been isolated from natural sources, they have all been modified α-amino acids: 6-diazo-5-oxo-L-norleucine (2, DON),1) azaserine (3),2) alazopeptin (4, L-Ala-DON-DON),3) duazomycin A (5, N-acetyl DON),4) N-(L-Alanyl)azaserine (6),5) and O-[(3R)-2-diazo-3-hydroxybutyryl]-L-serine (7, thrazarine).6,7) Other diazo compounds recently reported include lagunamycin (8), a 5-lipoxygenase inhibitor from Streptomyces sp. AA0310,8) and the kinamycins A ~ D (9), antibiotics originally isolated from Streptomyces murayamaensis.9,10) We report here the structure of cremeomycin (1), which shares the diazoketone functionality and cytotoxicity, but represents a completely novel structure in this class of compounds.