A method which uses 1-naphthylisocyanate as an HPLC precolumn derivatization reagent for amino acid analysis is described. Derivatization is carried out by adding the isocyanate dissolved in dry acetone to a buffered amino acid solution followed by extraction of the excess reagent with cyclohexane. The resulting naphthylcarbamoyl amino acids are stable and highly fluorescent, with excitation maxima at 238 and 305 nm and an emission maximum at 385 nm, for most amino acids. Ultraviolet detection near 222 nm, the absorption maximum, can also be employed. HPLC procedures permitting the analysis of protein hydrolysates, brain extract, cerebrospinal fluid, and blood plasma are presented. The method is particularly suitable for auto-sampler procedures since samples can be derivatized and diluted in advance and stored at room temperature in the sampler while awaiting injection. Other advantages include high sensitivity, the possibility of recovering the derivatives from the column effluent, and the absence of a reagent peak in the chromatograms.