Plasminogen activator inhibitor 1 (PAI-1) is a glycoprotein, the primary inhibitor of t-PA and u-PA. It plays an essential role in controlling any excessive activation of fibrinolysis. It is present in plasma associated with vitronectin, in free form or associated with t-PA and in the alpha granules of platelets.

Fibrinolysis corresponds to the solubilization of the fibrinous thrombus by plasmin, an enzyme originating from plasminogen adsorbed to fibrin. Activation of plasminogen takes place by t-PA and u-PA. PAI-1 by inhibiting plasminogen activators, controls the degradation of fibrinous thrombus.
A decrease in fibrinolytic activity promotes the occurrence of thrombosis, while excessive fibrinolysis leads to hemorrhages.