Plasmin and kallikrein but not thrombin cleave purified histidine-rich glycoprotein (HRG), and heparin binding inhibits the proteolysis of HRG. To assess the proteolysis of HRG in plasma, immunoaffinity chromatography was used to isolate HRG from human plasma samples and the extent of protein cleavage was determined after electrophoresis under reduced, denaturing conditions. In blood drawn into streptokinase or into urokinase, HRG (78 kDa) was degraded producing peptides ranging in apparent molecular weight from 67 to 9 kDa. In patients undergoing thrombolytic therapy almost no intact HRG remains after 30 minutes, but the levels of circulating HRG are unchanged, indicating that cleaved HRG is not quickly or extensively removed from the circulation.