Plasma proteins which interfere with blood coagulation have often been described in patients with systemic lupus erythematosus (SLE). The most frequent type interferes with the conversion of prothrombin to thrombin and thus prolongs the prothrombin time. Infrequently, SLE patients exhibit anticoagulants which appear to block the earlier stages of coagulation such as those involving factor VIII or the formation of activated factor XI (factor XIa). The anticoagulant reported here was studied by means of a sequential clotting system utilizing crude coagulation factors and was noted to interfere with the action of activated plasma thromboplastin antecedent (PTA) during the activation of factor IX. This anticoagulant was found in gamma-globulin-rich ethanol fractions of plasma. After gel filtration, it was found principally in fractions containing IgM globulins but also, to a lesser extent, in IgG-rich fractions. In this respect, it is similar to anticoagulants reported in certain other cases of SLE. Attempts to confirm the immunoglobulin nature of the anticoagulant by immunoabsorption were, however, inconclusive.