Amino acid levels of biological fluids are usually determined by the classical ion exchange procedure. Although the gas chromatographic method offers the advantage of higher sensitivity and speed of analysis, apparent difficulties in achieving accuracy and precision similar to the ion exchange procedure prevented its acceptance as a routine method. The main problems associated with gas chromatographic amino acid analysis were overcome by application of a novel approach called enantiomer-labelling: the optical antipode to each L-amino acid is added to the sample prior to the clean-up and serves as an internal standard having identical chemical properties. The enantiomers of all amino acids are separated by gas chromatography on capillaries coated with a thermally stable, chiral stationary phase. The shortcomings previously inherent in the gas chromatographic method are thus eliminated. Accuracy and precision of the new procedure are equal or better than of the classical ion exchange method.