A water-soluble metabolite of indole-3 acetic acid is formed by certain strains of Pseudomonas sacastanoi. This metabolite has been isolated from culture filtrates of this organism and purified by adsorption onto charcoal and by ion-exchange chromatography on sulfonic acid and quaternary ammonium resins.Hydrolysis of this isolated material by cationexchange resin and barium hydroxide yielded indole-3 acetic acid and lysine. Acid hydrolysis yielded lysine which was decarboxylated by L-lysine decarboxylase. The specific rotation of the isolated lysine indicates that the compound is the L isomer. Acid hydrolysis of the dinitrophenyl derivative of the unknown compound yielded a-dinitrophenyllysine. These results suggested that the unknown compound was indole-3-acetyl-~-~ lysine. An authentic sample of the latter compound was prepared by reaction of indole-3-acetic carbonic half-ester anhydride with the copper complex of Llysine and decomposition of the resulting copper derivative with hydrogen sulfide. The synthetic compound and the natural material showed identical infrared and ultraviolet spectra, melting points, and gave identical dinitrophenyl derivatives. Thus, the unknown compound was conclusively identified as indole-3-acetyl-~-~-lysine.