Platelet count, platelet serotonin uptake and platelet serotonin content were analysed in 21 subjects consisting of 11 patients of evolving acute myocardial infarction (AMI) and 10 matched healthy controls before and after administration of streptokinase. Platelet counts were significantly reduced in AMI with a subsequent rise following thrombolysis. Platelet 5-HT uptake was also significantly increased in AMI and following thrombolysis, it showed a trend towards normalization. Platelet 5-HT content was significantly increased in AMI with no further significant change following thrombolysis. The results suggest that platelet activation as revealed by reduced platelet count and altered platelet serotonin kinetics, occurs in AMI and this activation is inhibited following thrombolysis. Further, it is also apparent that it is not the reperfusion but the thrombolytic agent that is responsible for inhibition of platelet activation.