A 10-year-old boy had a severe lifelong hemorrhagic disorder that had necessitated more than 50 hospitalizations. Laboratory examination showed prolonged bleeding, clotting, partial thromboplastin, prothrombin, and thrombin times. These findings were due to a potent inhibitor of the thrombin-fibrinogen reaction. This inhibitor was similar to heparin in that it acted immediately and did not interfere with the coagulant activities of certain venoms. It differed from heparin in not being adsorbed to barium citrate or neutralized by protamine sulfate. The inhibitory effect was found in the alpha1-globulin fraction. It was identified immunologically and functionally as a double-banded alpha1-antitrypsin of a previously unreported phenotype. The inhibitory effects were depressed by trypsin and heterologous anti-alpha1-antitrypsin.