A method for quantitative analysis of protein amino acids using isotope ratio gas chromatography-mass spectrometry computer techniques is described. The amino acids are analysed as their N-trifluoroacetyl-n-butyl esters and the quantitation is accomplished by the inclusion of deuterated amino acid mixtures of known composition. The inclusion of internal standards prior to protein hydrolysis provides a method with high precision (coefficient of variation less than 5%). The method, which requires 100 microgram and can be performed in just over 2 h, when tested on insulin gave results which agreed well with the known composition of the protein and with simultaneous analysis on ion exchangers. Thus, the technique offers an alternative to automatic amino acid analysers (utilizing ion exchangers) for quantitative amino acid analysis.