The plasma beta-thromboglobulin level was studied serially in 14 patients with acute myocardial infarction. In 7 patients the plasma beta-thromboglobulin was initially high. The most severely affected patients had the highest values. Three patients with a mild or moderate clinical course had a normal beta-thromboglobulin level at every stage. The remaining patients had a distinct pattern in the evolution of the plasma beta-thromboglobulin values. The initial phase was followed by a uniform fall during the first days and then by an increase. The highest values were in many cases measured at discharge. Platelet counts showed a similar pattern. Signs of activation of platelets were seen before changes in coagulation and fibrinolysis.