Although several antitumor diazo ketones have been isolated from natural sources, they have all been modified a-amino acids: 6-diazo-5~oxo-L-norleucine (2, DON),1* azaserine (3),2) alazopeptin (4, L-Ala-DON-DON),3) duazomycin A (5, 7V-acetyl DON),4) 7V-(L-Alanyl)azaserine (6),5) and O-[(37?)-2-diazo-3-hydroxybutyryl]-Lserine (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.