The effects of gel-filtered platelets on euglobulin clot lysis time (ECLT) were analyzed to elucidate the possible role of platelets in thrombolysis. Gel-filtered platelet-supplemented ECLT (plt-ECLT) was significantly shorter than ECLT without platelets (regular ECLT). Abciximab, anti-glycoprotein IIb/IIIa (GPIIb/IIIa) antibody, and cytochalasin B nullified the enhancement of ECLT by platelets, and increased plt-ECLT beyond regular ECLT. When gel-filtered platelets were used after disruption, ECLT was not shortened but rather became longer than regular ECLT, probably due to natural fibrinolysis inhibitors released from platelets. Therefore, for platelets to enhance fibrinolysis, intact cell structure and cytoskeletal reorganization after thrombin stimulation is required. Various GPIIb/IIIa antagonists prolonged plt-ECLT. The concentrations of GPIIb/IIIa antagonists required to prolong plt-ECLT, were varied. Interestingly, the effects of these antagonists were independent of their ability to inhibit thrombin-induced platelet aggregation, but dependent on their ability to induce clot retraction. T-250, a GPIIb/IIIa antagonist, had the smallest effect on plt-ECLT. These drugs do not affect regular ECLT or tissue plasminogen activator (tPA)-catalyzed Glu-plasminogen activation in the presence of thrombin-activated platelets. Although their overall effect on thrombolysis is inhibitory, platelets could promote fibrinolysis through a GPIIb/IIIa-dependent mechanism.