The enantioseparation of standard mixtures composed of protein DL-amino acids was performed by reversed-phase high-performance liquid chromatography of the corresponding diastereomeric isoindolyl derivatives, formed by automated precolumn derivatization with o-phthaldialdehyde (OPA) and a series of N-acyl-L-cysteines(Acyl-Cys). A photodiode-array detector, operating at 338 nm, was used for detection. In order to evaluate systematically the influence of the structures of the acyl group in the chiral thiol reagents, a series of novel N-acyl-L-cysteines was synthesized [acyl = n-butyryl, isobutyryl (i-But), pivaloyl, benzoyl) and the chromatographic behaviour of the diastereomers formed was compared with those of already known reagents, N-acetyl-L-cysteine and N-ter.-butyloxycarbonyl-L-cysteine. All Acyl-Cys derivatives of DL-amino acids were resolved. In particular, i-But-Cys gave the highest resolutions for most of the amino acid enantiomers in comparison with the other Acyl-Cys. Investigation of yoghurt using OPA-acetyl-Cys demonstrated the applicability of the method to a complex food matrix and the occurrence of D-Asp, D-Glu and D-Ala in this dairy product.