High molecular weight kininogen (HK) is an abundant plasma protein that plays a central role for the function of the kallikrein⁄kinin⁄kininogen sys-tem. Thus, cleavage of HK by kallikrein liberates bradykinin, which stimu-lates vascular repair and a two-chain protein, activated HK (HKa), which induces apoptosis in proliferating endothelial cells.