Plasminogen activator inhibitor type-1 (PAI-1), a member of the serine protease inhibitor (serpin)superfamily, is an important protein in the regulation of fibrinolysis. PAI-1 is unique among the serpinsbecause of its functional and conformational flexibility. PAI-1 is the most important physiologicalinhibitor of both tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA). Increased PAI-1 levels are associated with thrombotic events and is an established risk factor forcardiovascular diseases. The active conformation PAI-1 inhibits its target proteinases by the formationof a stable, inactive complex. Although PAI-1 is synthesized as an active molecule, it convertsspontaneously to an inactive, latent form that can be partially reactivated by denaturing agents. Inaddition, a third conformation reacting as a non-inhibitory substrate towards various target proteinaseshas been identified.The epitope of monoclonal antibody MA-33H1F7 is predominantly composed of three residues(Lys154/Glu130/Arg131), positioned virtually linearly in the three-dimensional structure. The epitope of theantibody does not cover the complete alpha-helix F and turn connecting alpha-helix F and beta-strands3A, but is restricted to the hinge region between alpha-helix F and the main part of the PAI-1molecule.The monoclonal antibody MA-33H1F7 is a ‘switching’ antibody, capable of inducing a non-inhibitorysubstrate form of PAI-1. It was shown to inhibit PAI-1 in a dose dependent manner.