Bismuth subgallate is an effective agent in preventing hemorrhage after adenotonsillectomy. The experiments described demonstrate that this may occur through the activation of Hageman factor by this agent. Bismuth subgallate shortened the clotting time of whole blood, an action localized to an effect on the early steps of the intrinsic pathway; bismuth subgallate did not accelerate the thrombin time or prothrombin time of normal plasma, but could be substituted for kaolin as an activator of coagulation in assays of the partial thromboplastin time. The action of bismuth subgallate was localized to an effect on Hageman factor. It did not induce coagulation of plasma samples deficient in any of the recognized factors participating in the intrinsic pathway of thrombin formation, but it shortened the clotting time of plasma deficient in factor VII, a component of the extrinsic pathway. Evidence was obtained that Hageman factor exposed to bismuth subgallate corrected the defect of Hageman factor-deficient plasma and acquired amidolytic properties in the absence of other clotting factors. These studies provide a rationale for the hemostatic properties of bismuth subgallate.