The amino acid analysis method using precolumn phenylisothiocyanate (PITC) derivatization and liquid chromatography was modified for accurate determination of methionine (as methionine sulfone), cysteine/cystine (as cysteic acid), and all other amino acids, except tryptophan, in hydrolyzed samples of foods and feces. A simple liquid chromatographic method (requiring no derivatization) for the determination of tryptophan in alkaline hydrolysates of foods and feces was also developed. Separation of all amino acids by liquid chromatography was completed in 12 min compared with 60-90 min by ion-exchange chromatography. Variation expressed as coefficients of variation (CV) for the determination of most amino acids in the food and feces samples was not more than 4%, which compared favorably with the reproducibility of ion-exchange methods. Data for amino acids and recoveries of amino acid nitrogen obtained by liquid chromatographic methods were also similar to those obtained by conventional ion-exchange procedures.