• Platelet monoamine oxidase (MAO) activity levels were determined before and 180 minutes after ingestion of ethyl alcohol in 30 healthy men aged 21 to 25 years. The subjects included 15 men with alcoholic first-degree relatives who were matched by demography, height-weight ratio, and drinking history with 15 control subjects who had no family history of alcoholism. There was a nonsignificant trend toward lower platelet MAO activities at baseline and after ethyl alcohol ingestion in the group with alcoholic relatives when compared with the control subjects who had no family history of alcoholism. With an arbitrary MAO cutoff of 5.24 nmole/mg of protein per hour, eight of the 15 subjects with alcoholic relatives and 12 of the 15 without alcoholic relatives were correctly identified. However, because of the number of false-positive and false-negative findings, the results have limited clinical usefulness.