Activated high molecular weight Hageman factor (75 Kd) and Hageman factor carboxy-terminal fragments both formed complexes with purified C1(-)-inhibitor, but the Hageman factor fragments appeared to have a higher affinity for the C1(-)-inhibitor than activated Hageman factor. Therefore, the clot-promoting activity of activated Hageman factor might be relatively unimpaired if Hageman factor fragments are also present. Normal C1(-)-inhibitor was cleaved by Hageman factor fragments. Clot-promoting activity was not generated in Hageman factor by exposure to Hageman factor fragments, nor was Hageman factor cleaved by Hageman factor fragments. When Hageman factor was cleaved by streptokinase-activated plasminogen, a 40 Kd fragment was released. In contrast to their interactions with other proteinases, which are blocked by normal C1(-)-inhibitor, Type II C1(-)-inhibitors from plasmas of affected members of eight different kindred with this form of hereditary angioneurotic edema all inhibited the specific coagulant activity of activated Hageman factor to some degree. They did not all form complexes with activated Hageman factor that were stable during sodium dodecyl sulfate-polyacrylamide gel electrophoresis.