BACKGROUND: Fibrinolytic therapy represents a widely available and effective treatment modality for ST segment elevation myocardial infarction (MI). Its overall benefit is attenuated by a high incidence of coronary arterial reocclusion. METHODS/RESULTS: Human umbilical vein endothelial cells (HUVEC) were incubated with unfractionated heparin (1.5 U/ml) (to provoke tissue factor pathway inhibitor [TFPI] release) followed by the addition of varying concentrations of alteplase (recombinant tissue plasminogen activator), plasminogen, their combination or plasmin alone. In the presence of 20% TFPI-depleted human plasma, there was a concentration-dependent decrease in TFPI levels following incubation with alteplase (28% reduction at 200 ng/ml; P < 0.01); 37% reduction at 1000 ng/ml (P < 0.001). Similar effects were observed for alteplase combined with plasminogen (P < 0.001), plasmin alone (P < 0.001) and with HUVEC incubated with low concentrations of plasmin (10 ng/ml) prior to heparin exposure. CONCLUSIONS: Plasmin, a non-specific protease, degrades vascular endothelial cell (constitutive) TFPI and heparin-releasable TFPI, attenuating an important pathway of vascular surface thromboresistance and potentially contributing to coronary arterial reocclusion after fibrinolytic therapy.