Summary: Platelet function has been examined in humans during periods of enhanced fibrinolysis induced by streptokinase or occurring spontaneously. The degree of lytic activity has been assessed by thrombin times, fibrinogen levels and measurement of fibrin degradation pro-ducts in the serum. The bleeding time was usually normal although there was a tendency for prolongation during the course of prolonged lysis or in the presence of profound hypofibrinogenaemia. Platelet adhesiveness to glass beads and aggregation with adenosine di-phosphate and collagen were usually normal even in the presence of markedly elevated levels of degradation products. Platelet aggregation with thrombin was impaired and the abnormality could be corrected by increasing thrombin concentrations. The results suggest that the major haemostatic defect during enhanced lysis results from the anti-thrombin action of degradation products and that there is little decrease in platelet aggregation as long as fibrinogen levels are not excessively reduced.