We identified a circulating, heparin-like anticoagulant in a patient with Wegener's granulomatosis. Routine coagulation studies revealed a significant prolongation of the coagulation time, thrombin time, prothrombin time and activated partial thromboplastin time. Thrombin time and activated partial thromboplastin time failed to improve in a 1:1 mixture with pooled normal plasma. Both test were corrected in vitro by addition of protamine sulfate or toluidine blue and in vivo by intravenous administration of protamine sulfate. Recognition of the existence of this or other similar inhibitors in bleeding patients is of value because of the possibility of treatment with protamine sulfate, which neutralizes the anticoagulant.