Clark''s Ca method was checked against ashing methods and the amount of Ca was found to be the same within experimental error, in serum, citrated plasma and heparinized plasma, if the oxalate was allowed to stand in contact with the plasma or serum for sufficient length of time. It was found that the ether-extractable lipids held some Ca in combination with them, but not all Ca soaps act as thrombin. Throm-bin is probably a Ca-"tissue juice" compound which alters the sol fibrinogen to the gel fibrin. Prothrombin, if such exists, is any potential source of tissue extract (broken tissues, platelets or white blood cells) and not a separate chemical entity of the circulating blood. Rettger''s work regarding the recalcification of fluoride blood was confirmed. Fluoride plasma will clot upon recalcification if the Ca is not added in excess. Vine''s results showing that 3 or 4 times the amount of oxalate necessary to precipitate the normal amount of blood Ca must be added to blood to prevent its clotting, were confirmed. Thrombin is inactivated by means of oxalate. This is due to the removal of Ca and is not merely a salt effect. It seems impossible to remove Ca from thrombin by dialysis. Fibrin ordinarily contains Ca. but this Ca may be removed by washing. The Ca of fibrin probably belongs to the thrombin. Lowering of surface tension of blood is probably the causative factor concerned in the rupturing of platelets in shed blood.