[en] The authors have developed a method for determination of the specific activity of radio-labelled leucine by collection of the appropriate peak of the orthophthaldehyde (OPA) derivative from a C18 reversed phase HPLC column. The derivatizing reagent comprised 0.4 M OPA-0.45 M 3-mercapto-proprionic acid in MeOH combined with 1/2 vol 1M KBO₃-0.2 M Na₄EDTA or 1M KBO₃-1M K₂CO₃ immediately prior to mixing with 1 vol amino acid solution. The authors conclude that the method is more rapid than ion exchange chromatography, utilizes standard chromatographic equipment without effluent stream splitting, and is not limited in principle to essential amino acids