1. The results of the one-stage prothrombin consumption test, performed in a group of patients with various abnormalities of the hemostatic mechanism, are reported. It appears that the test accurately identifies those conditions such as hemophilia and thrombocytopenic purpura which involve a deficiency of available thromboplastin. Other conditions examined in which a hemorrhagic tendency may develop (hypoprothrombinemia, polycythemia vera, pseudohemophilia) showed a normal consumption of prothrombin. In the hypoprothrombinemia due to dicumarol therapy, the limited amount of available prothrombin appears to have been well utilized during the process of coagulation.
2. It appears that correction of the results obtained for accelerator activity of serum is hardly ever necessary, since this effect is not of such magnitude as to modify the results of the prothrombin consumption test to any extent. A formula for such correction is given.