High molecular weight kininogen (HMWK or HK) is a circulating plasma protein which participates in the initiation of blood coagulation, and in the generation of the vasodilator bradykinin via the Kallikrein-kinin system. HMWK is inactive until it either adheres to binding proteins beneath an endothelium disrupted by injury, thereby initiating coagulation; or it binds to intact endothelial cells or platelets for functions other than coagulation.