An elderly woman with a bleeding problem associated with prolonged PTT, PT and thrombin time but with normal snake venom time was found to have very high levels of plasma antithrombin. The antithrombin activity was isolated and characterized and found to be an IgG which directly inhibited thrombin and also precipitated prothrombin. The concentration in plasma of the gamma-globulin inhibitory activity was equivalent per ml to 80 NIH units of thrombin. Corticosteroid and immunosuppressive treatment reduced the level of the inhibitor with remission of bleeding symptoms.