Advances in liquid chromatography have brought about the development of new techniques in amino acid analysis which take full advantage of precolumn derivatization procedures. Using phenylisothiocyanate as the reagent, detection limits under 1 pmol can be routinely achieved, allowing the analysis of submicrogram protein samples. Analysis times as short as 10 min for samples after hydrolysis and 1 h for physiologic samples are possible. Accurate, reproducible quantitation of amino acids can be obtained from complex matrices such as plasma, urine, feed, and food samples. This level of performance and flexibility gives the analyst the first realistic alternative to ion-exchange analysis without compromising desirable features of the traditional methodology.