The influence of 400 mM ethanol on the activity of monoamine oxidase type B (MAO-B) of blood platelets has been studied in vitro in 30 alcoholic patients and 30 healthy volunteers. Benzylamine was used as a substrate for MAO-B. MAO-B inhibition by ethanol was higher in alcoholics compared to healthy volunteers. The higher vulnerability of MAO-B of alcoholics inhibiting action to of ethanol may be one of the mechanisms underlying some symptoms of alcoholism.