Denamycin is a new antibiotic obtained from the cultured filtrate and mycelium of Streptomyces sp. No. 8756-CC2. It is purified on silica gel column chromatography followed by crystallization from organic solvents. It shows inhibitive activities against the nucleic acid syntheses in the bacterial cells and against the growth of ascites or solid form of tumor in mice. It forms yellow needles melting at 226-228°C, and has an optical rotation of [α]20D +41° in methanol. The antibiotic shows ultraviolet absorption maximum at 311 mμ (E1%1 cm 520) in ethanol, and is very unstable and degraded into several compounds under the atmospheric condition and ultraviolet light. © 1969, JAPAN ANTIBIOTICS RESEARCH ASSOCIATION. All rights reserved.