Background Platelets play a pivotal role in the pathogenesis of acute myocardial infarction, as well as in the occurrence of coronary artery reocclusion and bleeding events. Therefore, the success of fibrinolytic therapy may be dependent on its direct effects on platelets. Methods and Results We sought to determine how tenecteplase (TNK) and alteplase (tPA) affect platelets in vitro in human volunteers and ex vivo by use of patient data from the Assessment of the Safety and Efficacy of a New Thrombolytic Agent (ASSENT-2) trial. For the in vitro studies, whole blood from 9 healthy volunteers was incubated with 30 mg TNK and 60 mg tPA. Platelet function was measured by conventional aggregometry, bedside point-of-care devices, and sensitive flow cytometry techniques. For the ex vivo study, 41 patients were selected from the ASSENT-2 trial: 21 had received TNK and 20 had received tPA. Each patient underwent 7 serial blood draws every 30 minutes for 3 hours. Levels of platelet endothelial cell adhesion molecule-1 (PECAM-1), vascular cell adhesion molecule-1 (VCAM-1), P-selectin, β-thromboglobulin, platelet factor 4, thromboxane, and prostacyclin were measured by enzyme-linked immunosorbent assay. Significant inhibition of conventional and whole blood aggregation and reduced platelet function by point-of-care analyzers were observed for both agents, but mostly in the TNK-treated samples. After incubation with TNK, flow cytometry revealed decreased expression of glycoprotein IIb/IIIa, PECAM-1, vitronectin receptor, CD151, and formation of the platelet-monocyte aggregates. Serial samples from patients in the ASSENT-2 trial showed a significant decrease of soluble platelet-endothelial biomarkers in the TNK group. There was a trend toward decreased platelet function characteristics with tPA; however, these differences were much smaller than those observed with TNK. Conclusions Both tPA and TNK were shown to affect platelet function in human volunteers and in patients with AMI early after thrombolysis. The antiplatelet properties of TNK were shown to be more profound than those of tPA. These findings are relevant to ongoing investigations of combination therapy with fibrinolytic and antiplatelet agents in patients with AMI. (Am Heart J 2003;145:636-42.)