A modified radiometric procedure is developed for estimation of amine oxidase activity. The method avoids withdrawal of samples in order to measure the radioactivity of reaction products. The enzymatic reaction, extraction of the products and measurement of the radioactivity were carried out in the same scintillation vials. Although the experimental error was shown to be higher as compared with the conventional procedure it did not interfere in the practical use of the modified procedure up to 0.1 microCi concentration of substrate per a sample. The modified procedure exhibited high efficiency in estimation of the amine oxidase activity in soluble fraction of human placenta. The variation coefficient of the method was 5.6%.