A new antibiotic, tsushimycin, was isolated from cultures of a Streptomyces strain. The antibiotic is an acidic acylpeptide related to the amphomycinglumamycin group of antibiotics. However, fatty acid analysis by means of gas-liquid chromatography revealed that the antibiotic contains isotetradecenoic acid which has not been found in known antibiotics. The constituent fatty acids as well as the amino acid composition were compared with those of the above group of antibiotics.