We investigated ex vivo spontaneous platelet aggregation (SPA) in platelet-rich plasma in 37 patients with acute myocardial infarction. It occurred in about 50% of subjects receiving heparin after streptokinase treatment, while it rarely took place in patients who did not receive either streptokinase or heparin and in those treated with streptokinase alone. The study of patients receiving heparin for deep vein thrombosis suggested that SPA may derived from adenosine diphosphate released from platelets during sample handling. We suggest that heparin infusion may facilitate ex vivo platelet activation and that this mechanism is operative in patients with acute myocardial infarction who have undergone thrombolytic therapy.