A simple method for the determination of diamine oxidase (DAO) activity is described. The method is based on oxidation of histamine with DAO, formation of histamine-copper (II) chelated cation from the remaining histamine, and extraction of the ion associated with tetrabromophenol-phthalein ethyl ester. A linear calibration curve between absorbance and DAO activity was obtained for DAO activity up to 250 mU/ml. The inter-assay coefficient of variation was 2.12% with a recovery of 101.7% at the DAO activity of 123.5 mU/ml. A close correlation was found between the results of this method and the 2, 2'-azino-di (3-ethylbenzthiazoline-6-sulfonic acid) method.