Thrombin-activatable fibrinolysis inhibitor (TAFI) is a plasma carboxypeptidase that renders a fibrin-containing thrombus less sensitive to lysis. Since the role of TAFI in thrombus formation is still controversial in mice, our present study was designed to evaluate mice deficient in TAFI (TAFI−/−) on FeCl3-induced vena cava and carotid artery thrombosis. Parallel studies were carried out in wild-type mice using a potato carboxypeptidase inhibitor (PCI), a selective inhibitor of activated TAFI (TAFIa). Significant reduction in thrombus formation was observed in TAFI−/− mice (n = 8, P < 0.05 compared to wild-type littermates) but not in heterozygous (TAFI+/−) mice in 3.5% FeCl3-induced vena cava thrombosis. A similar effect was observed following treatment with 5 mg/kg bolus plus 5 mg/kg/h PCI in the same venous thrombosis model in C57BL/6 mice (n = 8, P < 0.01 compared to vehicle). No compositional difference was observed for the venous thrombi in TAFI−/− and wild-type littermates with or without PCI treatment using histological assessment. In contrast, neither TAFI deficiency nor treatment with PCI showed antithrombotic efficacy in the 3.5% FeCl3-induced carotid artery thrombosis model. In a tail transection bleeding time model, both TAFI deficiency and PCI treatment increased bleeding time up to 4.5 and 3.5 times, respectively, over controls (P < 0.05, n = 8). Similar ex vivo fibrinolytic activities were demonstrated for both TAFI deficiency and PCI treatment as enhanced lysis of thrombin-induced plasma clots and lysis of whole blood clot in a thrombelastograph. These data provide direct evidence for the role of TAFIa in vena cava thrombosis without the addition of exogenous thrombolytic in mice. The strong ex vivo fibrinolytic activity of TAFI deficiency or TAFIa inhibition by PCI provides a biomarker of TAFIa inhibition that tracks in vivo antithrombotic efficacy.