1. Distilled water dialyzes through the collodion tube and causes hemolysis. Clotting of the hemolyzed blood in the collodion tube occurs later. 2. Bile salts accelerate the appearance and development of white thrombi in the heparinized animal. The masses of white thrombi are very loose, soft, fragile and easily broken into clumps of platelets. They are poor in fibrin or the fibrin is abnormal, since they have less retractility than those obtained from the use of physiologic sodium chloride solution or calcium chloride or serum. Despite the increase in number and size of white thrombi, normal clotting does not occur. 3. When the collodion tube is surrounded by 1 or 2 per cent d-glucose solution, tiny and numerous white thrombi appear as radiating figures. The masses of white thrombi are rather loose and fragile. The clotting time of the heparinized blood does not appear to be shortened. 4. When a 1 to 2 per cent solution of calcium chloride is used as the dialyzing fluid outside the collodion tube, or when it is injected into the circulation, the formation of white thrombi is accelerated. They grow very rapidly. In spite of the action of heparin, the white thrombi formed are not so fragile as when bile salts are placed outside of the collodion tube. Fibrin seems to form easily. Obstruction of venous cannula takes place speedily and if the clots in the cannula are not removed, the white thrombi in the collodion tube remain small and become red by sedimentation of red cells. 5. Intravenous use of 10 per cent solution of magnesium sulfate without heparin retards the coagulation of circulating blood and permits the blood to flow through the extracorporeal loop from three to four times as long as normal. The formation of white thrombi, as well as red, is retarded. Magnesium sulfate (1 per cent) in physiologic sodium chloride solution placed outside the collodion tube markedly retards the formation of white and red thrombi in the heparinized animal. Magnesium sulfate (10 per cent), 50 mg. for each kilo of body weight each hour, administered intravascularly in the heparinized animal definitely prevents the first stages of thrombosis, and consequently prevents clotting. 6. It is possible by the combined use of adequate amounts of magnesium sulfate and of heparin intravenously to prevent all stages of thrombus formation for from 1 to 3 hours.