A simple colorimetric method for measuring the total activity of leucine aminopeptidase (EC 3.4.11.1) and arylamidase (EC 3.4.11.2) in serum was developed. The assay method is based on the following series of reactions : leucine aminopeptidase and arylamidase hydrolyze a substrate, L-leucyl-L-2-(4-hydroxyphenyl)glycine, to liberate 2-(4-hydroxyphenyl)glycine. The compound is then converted into 4-hydroxybenzaldehyde by laccase-catalyzed oxidative decarboxylation. Finally, the 4-hydroxybenzaldehyde produced is colorimetrically measured by treatment with 2,4-dinitrophenylhydrazine to evaluate the total activity of these aminopeptidases.