During the process of screening actinomycetes for novel antimicrobial substances, a strain of Streptomyces humifer was found to produce a previously unreported antibiotic, A51493A, with in vitro activity against Gram-positive bacteria. Spectroscopic analysis has established that A51493A is structurally related to the anthra-cyclinones rubiflavinone C-l and C-21}. Culture A51493 grew well on both complex and defined media. Trace amounts of white aerial hyphae were occasionally produced. Fragmentation similar to that described for S. humifer occurred when the culture was grownin liquid media2). A51493 utilized glucose, galactose, fructose, inositol, mannitol, and raffinose. Starch was hydrolyzed. Nomelanoid pigments were produced. Sugars present in the whole cells were glucose and ribose. Hydrolyzed whole cells contained LL-diaminopimelic acid. Using methods described by Shirling and Gottlieb3), culture A51493 was determined tobe a strain of S. humifer.