1. A case is reported of hemorrhagic diathesis in a woman in which the presence of an anticoagulant in the patient’s blood and plasma could be demonstrated.
2. The anticoagulant was shown to be active in dilutions up to 1:350. It retained its potency after heating to 61 C. for 10 minutes and after storage, either in a refrigerator or at room temperature, for twenty-four hours.
3. The anticoagulant was not neutralized by protamine sulfate or by toluidine blue. Placental plasma corrected the clotting defect in vitro but was ineffective when administered intravenously to the patient.
4. Evidence is presented suggesting that the second and third stages of coagulation in this case are normal, that the patient’s plasma has antithromboplastic activity, and that the anticoagulant may be antithromboplastin.