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.Tsushimycin was isolated from the culture broth of a Streptomyces strain Z-237 by ^-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.Tsushimycin was purified by charcoal decolorization and column chromatography on silica gel. A free acid form of tsushimycin was obtained as a colorless amorphous powder, m. p. 230~240°C (dec). A possible molecular formula C59H9302oN13-6H20 was indicated from elemental analysis and molecular weight determination. The optical rotation was [a]l3 +ll.9+0.5° in methanol, +12.2+0.5° in ethanol, and -5.6+0.5° in M phosphateThe antibiotic is soluble in lower alcohols, acid and alkaline water, but not or slightly soluble in acetone, ethyl acetate, chloroform, ether and petroleum ether. Amphoteric nature with an isoelectric point at approximately pH 3.0 was indicated by paper electrophoresis carried out in several buffer solutions of different pHs. It shows weakly positive ninhydrin and biuret reactions and decolorizes potassium permanganate and bromine solutions. In the ultraviolet absorption spectrum measured in methanol, only an absorption at 207 mju was observed. The infrared absorption spectrum on a KBr tablet showed the following absorption bands (cm"1) : 3300, 3040, 2920, 1725 (carboxyl), 1655, 1530 (peptide bond), 1450, and 1015. The acid hydrolysate of tsushimycin was subjected to two-dimensional paper chromatography and automatic amino acid analysis. The following amino acids were found : aspartic acid, glycine, proline, pipecolic acid, valine, and trace amounts of ammonia as well as two unknown amino acids, which, however, can be deduced to