High-performance liquid chromatographic (HPLC) analyses of amino acids, a so-called amino-acid analyzer based on the classical cation-exchange chromatography, 1,2 are essential for routine work in biological, biomedical, and clinical research laboratories. The modern cation-exchange HPLC is established by using a high-resolution cation-exchange column of highcapacity in mequiv./mL order of magnitude, with several buffer eluents (often with citrate buffers) at higher concentrations (often 0.2 M). The delivery of the eluent must be controlled by means of a complex gradient program, including parameters, such as pH, ionic strength, and temperature. Although such specialized amino-acid analyzers, commercially available from several manufacturers, can provide excellent performance for separations, they are usually expensive and seem to be unsuitable for general use. An alternate way to analyze amino acids based on anion-exchange chromatography has recently been proposed. 3,4 The separations have been good and satisfactory, but requiring an established instrument with a complex ternary gradient elution program.Ion-pair chromatography techniques by the usual HPLC systems have also been proposed by several researchers, 5,6 but they have not yet found general acceptance.On the other hand, low-capacity cation-exchange separations of amino acids have been proposed, 7,8 using a conventional HPLC instrument with an acid eluent of lower ionic strength, which can permit the use of UV detection alternative to fluorescent detection. Such a simplified amino-acid separation system can reduce the analytical costs. In our previous work, 7 a low-capacity cation-exchange resin, pellicularly sulfonated polystyrene-divinylbenzene (PS-DVB) resin of 4% crosslinkage, provided by courtesy of a manufacturer as a trial product, was successfully used for the separation of amino acids. The cation exchanger used has shown good selectivity for amino acids, but has given poor resolutions because of the larger resin diameter (11 µm in average) and the lower crosslinkage, leading to low column efficiency.In general, concentrated sulfuric acid is often used to sulfonate a PS-DVB base polymer. However, it seems to be difficult to control the ion-exchange capacity, especially in µequiv./mL levels. In addition, because the sizes of the base polymers used in several studies [9][10][11][12] have been relatively large, exceeding 10 µm in diameter, it seems difficult to obtain highefficiency columns for amino-acid separations.Recently, sulfoacylated 13 macroreticular PS-DVB resins have been successfully used for the separation of multivalent metal cations, 14 in which size-exclusion beads of hypercrosslinked 5-µm polymers have been functionalized to an ion-exchange capacity of around 0.4 mequiv./mL. This kind of cationexchange resin was considered to be acceptable for the separation of amino acids, but the capacity was too large compared to those successfully used in our previous work. 7 Seubert and Klingenberg 13 have mentioned in their paper that the capa...