A patient who had received multiple transfusions for complications of acute hemorrhagic pancreatitis developed a potent factor V anticoagulant with bleeding due to defective hemostasis. Despite its potency, the anticoagulant disappeared within 15 days of its first manifestation. A second patient with adenocarcinoma of the colon developed an anticoagulant to factor V postoperatively after a single blood transfusion. The anticoagulants appeared to react stoichiometrically with factor V in normal plasma in vitro. They had the physicochemical properties of immunoglobulins, and their activity was neutralized by antihuman immunoglobulin antiserum. One anticoagulant appeared to be slightly more active against homologous than against autologous factor V, but it also inhibited heterologous factor V. Both anticoagulants progressively inactivated intrinsic prothrombin activator formed from normal reagents in the incubation mixture of the thromboplastin generation test, thus confirming that factor V is required for the effective action of the intrinsic prothrombin activator. Since the anticoagulants were immunoglobulins whose activity was consumed in their reaction with factor V, consumption of anticoagulant activity was used to detect factor V antigenic material in test materials. Human serum without factor V clotting activity was found to consume anticoagulant activity, i.e., to contain inactive factor V antigenic material. Plasma from two patients with hereditary factor V deficiency (parahemophilia) failed to consume significant anticoagulant activity. Thus, the lack of factor V activity in these patients represents a deficiency of factor V molecules rather than the synthesis of a defective molecule with impaired clotting activity.