Platelet MAO activity of 176 healthy volunteers was studied. Activity of the enzyme was found intraindividually stable, repeated measurements in the same individuals performed 3 weeks and one year later revealed only minimum changes. Platelet MAO activity was found to be significantly higher in females than in males. No age dependence of the measured values could be demonstrated. Enzyme activity values determined with different substrates showed a significant positive correlation. A significant activation of the enzyme was found after the addition of platelet poor plasma to the platelet preparation.