Steffimycin B, an anthracycline, has been isolated from cultures of Streptomyces elgreteus by solvent extraction and purified by silica gel chromatography. NMR and mass spectroscopy indicate the presence of a methoxyl group in the sugar moiety of steffimycin B not found in steffimycin, a related antibiotic. Steffimycin B is not active against bacteria in experimentally infected animals, but has shown some potential antitumor activity in an in vitro screen. Further testing is in progress.