Abstract.If diamine oxidase is allowed to act upon the diamine putrescine, a cyclic compound, δl‐pyrroline, will be formed, which in turn reacts with o‐aminobenzaldehyde to form a yellow compound. These reactions have been taken as a starting point for a photometric method for determination of diamine oxidase. If a solution containing o‐aminobenzaldehyde, putretrcine and a diamine oxidase preparation is incubated at 37° C a characteristic yellow colour is obtained. The intensity of the colour can be measured spectrophotometrically. The influence of pH and of the concentration of putrescine and o‐aminobenzaldehyde on the reaction has been tested. The wavelength which gives a maximal extinction has been determined. The variation of the extinction with increasing time of incubation has been studied. On the basis of these investigations a standard procedure is suggested. Simultaneously determinations of diamine oxidase in different dilutions have been carried out using this colorimetric method and the Warburg