Serum Ac-globulin is a clotting factor found in the globulin fraction of blood serum1, 2 which accelerates the change of prothrombin to thrombin. This factor is present in plasma in a much less active form, plasma Ac-globulin,3, 4, 5, 67 which serves as a precursor to serum Ac-globulin. The role of Ac-globulin in the total clotting reaction has been described as follows:1, 2 The clotting reaction is initiated by thromboplastin, which, in the presence of calcium ions, interacts with prothrombin to form thrombin. The thrombin in turn alters the plasma Ac-globulin so that it becomes serum Ac-globulin. The latter then intensifies the reaction between thromboplastin and prothrombin. Clotting is then accomplished by the action of thrombin on fibrinogen.In the developmental work on Ac-globulin, oxalated bovine plasma was used as a potent stable source of plasma Ac-globulin8 and bovine serum1 a stable source of serum Ac-globulin. In working with the sera of other species involving man, dog, and guinea pig, we hav...