The functional resistance of blood platelets, the reserve and active forms of histamine and serotonin, and the activity of monoamine oxidase and histaminase were studied in 5 healthy subjects, in 64 patients with acute, subacute and rehabilitation stages of myocardial infarction, and in 33 patients during frequent attacks of angina pectoris and after their total subsidence. On the grounds of the data obtained, the authors conclude that: (1) the drastically disturbed blood platelet resistance, megathrombocytosis, and marked decrease in the content of reserve amine forms in ischemic heart disease should be considered an additional risk factor; (2) agents which improve blood platelet vital activity and/or block the release reaction should be included in the therapeutic program for patients suffering from ischemic heart disease with heightened risk factors.