The reaction between primary amines and o-phthaldialdehyde in the presence of a thiol was exploited to measure the concentrations of 21 amino acids in serum by high-performance liquid chromatography with fluorescent detection. The method described here includes an automatic on-line precolumn procedure for derivatizing the amino acids, permitting full automation and avoiding problems due to time-dependent decay of fluorescence of the derivatives. The total analysis time is less than 60 min and limits of sensitivity are about 100 fmol. Proline, hydroxy-proline, and cysteine are not detected. Comparison with data generated by a standard amino acid analyzer shows this technique to be reliable.