A new crystalline antibiotic named neopluramycin, which inhibits growth of Gram-positive bacteria, leukemia L-1210 in mice and Yoshida rat sarcoma cells in tissue culture, has been isolated from the cultured broth of a strain of Streptomyces pluricolorescens. It was obtained as orange crystals, and analytical data and molecular weight determination are consistent with the empirical formula C40H50N2O10. Neopluramycin resembles pluramycin A, but is differentiated by its antibacterial spectrum, toxicity, thin-layer chromatography, and infrared absorption spectrum. An antibiotic produced by a strain of Streptomyces pluricolorescens was isolated from the cultured broth by solvent extraction. It exhibits inhibition against Gram-positive bacteria, Yoshida rat sarcoma cells and leukemia L-1210 in mice. Taxonomic studies of the streptomyces, isolation and characterization of neopluramycin are described in this paper. The antibiotic was differentiated from known antibiotics and named neopluramycin.