A high-performance liquid chromatographic system for the determination of amino acid and imino acid was developed using a two-step reaction with sodium hypochlorite and o-phthalaldehyde/N-acetyl-L-cysteine (OPTA/AcCys) reagent. This reagent improved the sensitivity in the analysis of proline presumably because the fluorophore is more stable to hypochlorite which has been used for the oxidative cleavage of the imino linkage. The use of OPTA/AcCys facilitated the detection of imino acids at the same concentration level as that of amino acids. The detection limit for all the amino and imino acids was a few picomoles. This detection system, together with cation-exchange chromatographic separation, was applied to the determination of amino and imino acids in biological samples.