A new antibiotic, tsushimycin, was isolated from cultures of a Streptomyces strain. The antibiotic is an acidic acylpeptide related to the amphomycin glumamycin group of antibiotics. However, fatty acid analysis by means of gas-liquid chromatography revealed that the antibiotic contains isotetradecenoic acids as well as the amino acid composition were compared with those of the acid which has not been found in known antibiotics. The constituent fatty above group of antibiotics. Tsushimycin was isolated from the culture broth of a Streptomyces strain Z-237 by n-butanol extraction at acid pH. The crude material contained two active substances which were separable on thin-layer chromatograms. The slower moving one was named tsushimycin. The faster moving one was a minor product and was judged to be a similar antibiotic from amino acid analysis.