Formation of thromboxane B2 (TXB2), a metabolite of the potent platelet-aggregating and vasoconstrictor agent thromboxane A2 (TXA2), during ADP-induced platelet aggregation was studied in 10 healthy men and in 10 male alcoholics during the 2-week period of detoxification. None of the alcoholics had anemia or thrombo-embolic disease. The platelets of the alcoholics were more sensitive for ADP and synthesized as much as triple the amount of TXB2 compared to those of the nonalcoholic donors. The effect was most striking during the rebound thrombocytosis and suggests that it could possible contribute to the increased incidence of various thrombotic diseases in the alcoholic.