A factor in serum which accelerates the conversion of prothrombin to thrombin; its relationship to the coagulation defect of thrombocytopenic blood.

The sera from thrombocytopenic blood show abnormally large residual prothrombin activity and small amounts of prothrombin conversion accelerator. The addition of normal platelets or thromboplastin corrects these abnormalities. In one subject the clotting defect persisted despite temporary remission of the thrombocytopenia consequent to splenectomy.