A simple colorimetric method for determining the activity of angiotensin 1-converting enzyme (ACE) was developed. The assay method is based on the following series of reactions : ACE hydrolyzes a tripeptide substrate, N-benzoyl-L-2-(4-hydroxyphenyl)glycyl-L-histidyl-L-leucine to liberate N-benzoyl-L-2-(4-hydroxyphenyl)glycine. The compound is then converted into 4-hydroxybenzaldehyde (HBA) through oxidative decarboxylation catalyzed by bilirubin oxidase from Myrothecium verrucaria. Finally, HBA is measured colorimetrically by treatment with 2,4-dinitrophenylhydrazine to evaluate the activity of ACE. The optimum pH of ACE for the substrate was 8.3, and the K(m) value was 0.072 mM. The formation rate of HBA was proportional to the ACE concentration, and the correlation coefficient was 0.997.