An extensive retrospective comparison was conducted of the long-term repeatability and consistency of amino acid analyses using phenylisothiocyanate and 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) precolumn derivatization. Amino acid standards were analyzed more than 130 times on more than 60 independent occasions by each of these two precolumn derivatization methodologies, during routine amino acid analysis procedures. Similar coefficients of variation were seen only when very freshly prepared derivatives were analyzed. When realistic aging for < 20 h was taken into account, the extreme stability of the AQC derivatives stood out. Chromatography of AQC derivatives using HPLC solvents prepared on a large scale provided the lowest coefficients of variation. The superiority of AQC over PTC methodology was clearly apparent.