A 78-year-old woman, who had received steroid therapy for four years, had strikingly prolonged prothrombin time (PT) and partial thromboplastin time (PTT). The presumptive diagnosis was chronic hemolytic anemia. Platelet count and functions, fibrinogen, and factor XIII assays were all normal, but other factors assayed abnormally low; Sia water-dilution test was positive. When water-insoluble protein was removed by centrifugation, coagulation factors became normal. Dissolution of this precipitate in normal plasma caused marked prolongation of PT and PTT and lower factor assays. Serum electrophoresis showed a homogeneous M spike and an anomalous IgM, λ-antigenic type in the γ-globulin zone at point of origin. Ultracentrifugation of serum and of the precipitate showed 10% S17 and almost 100% S17 components, respectively. Five other patients with water-insoluble paraproteins were tested; two were clot-inhibitory.