To provide a determining method in which a substrate is quickly decomposed by carboxypeptidase-A compared with substrate in conventional method, capable of rapidly accurately determining carboxypeptidase-A activity and useful for filed of clinical examination of pancreas diseases.

This method for determining carboxypeptidase-A activity comprises decomposing a dipeptide substrate represented by the general formula, R-CO-X-Y-OH in which N terminal is protected (X is a methionine residue or an amino acid residue having an aromatic ring or its derivative; Y is an amino acid residue having aromatic ring or its derivative; R-CO is a protecting group of amino group) by carboxypeptidase-A and reacting the produced H-Y-OH with an enzyme capable of determining the compound.