A practical method for the quantitative determination of protein amino acids by gas-liquid chromatography (GLC) is described. All of the common protein amino acids except arginine can be readily converted into their N-isobutyloxycarbonyl (N-isoBOC) methyl ester derivatives by a simple procedure involving isobutyloxycarbonylation with isobutyl chloroformate in aqueous medium, followed by methylation with diazomethane. Arginine was converted into N-isoBOC omithine methyl ester by treatment with arginase, followed by the above derivatization procedure. The resulting N-isoBOC methyl esters of the amino acids have good GLC properties. Complete resolution of the derivatives of 20 protein amino acids was achieved by using a dual-column system consisting of a 0.65 % Poly-A-1OlA column and a0.70 oA FFAP-Poly-A-1QlA (l:l, w/w) column. The reproducibility of response was found to be good for derivatives carried through the entire chemical and chromato,mphic procedure. The calibration graphs were linear and showed no statistical bias. The results of recovery experiments with synthetic mixtures containing known amounts of the amino acids were satisfactory, the recoveries ranging from 94.3 to 106.2%. I30