The biosynthetic origin of the amino acid moieties of enduracidin was investigated by feeding experiments with labeled compounds. Results of the incorporation and the distribution of radioactivity into the antibiotic revealed that glycine, L-serine, L-threonine, L-alanine, L-aspartic acid, L-ornithine and L-citrulline were incorporated into the corresponding amino acid moieties. Unique amino acids, enduracididine and its isomer with an imidazolidine ring, were derived from L-arginine, but not histidine. K1 (4-hydroxyphenylglycine) and K2 (3,5-dichloro-K1) moieties were derived from L-tyrosine. 36Cl-Sodium chloride was incorporated into the antibiotic in the early stage of fermentation.