TAFI, thrombin-activatable fibrinolytic inhibitor, (also known as carboxypeptidase U and plasma pro-carboxypeptidase B) is a 60kDa molecular weight glycoprotein (proenzyme form) found in human plasma that modulates fibrinolysis. This proenzyme is converted to an active form of molecular ratio 35kDa, TAFIa, after proteolytic cleavage by the thrombin / thrombomodulin complex.

TAFIa possesses carboxypeptidase activity with a preference for cleavage of lysine and arginine residues from the end of proteins. Modulation of fibrinolysis occurs when TAFIa cleaves the C-terminal arginine and lysine residues of partially degraded fibrin. Removal of arginine and lysine residues from fibrin inhibits the continued degradation of fibrin by tPA-activated plasmin. TAFI may play a central role in thrombosis and fibrinolysis due to its ability to delay fibrin clot lysis.