Factor XII (FXII, Hageman factor) is a 80kD, single chain glycoprotein that circulates in blood as an inactive zymogen. FXII plays an important role in blood coagulation, fibrinolysis, and kinin generation. When plasma comes into contact with negatively charged surfaces such as glass, kaolin, or dextran sulfate, FXII is bound to the surface and undergoes limited proteolysis by plasma kallikrein, resulting in two active enzyme forms, ɑ-FXIIa and β-FXIIa. Hereditary deficiencies in FXII are not associated with spontaneous or excessive bleeding. FXII inhibition also have protective role against arterial thrombosis and stroke, suggesting that FXII may be a target for antithrombotic therapy. This kit is used to quantify FXII in vivo.