The experiments here outlined show that urea, added to blood, produces at least 3 effects: (1) In low concentrations it accelerates clotting by the liberation of reserve supplies of throm-bin precursors, i.e., cephalin and prothrombin. (2) In higher concentrations (up to about 5 gm. per 100 cc.) it retards clotting by delaying the union of thrombin with blood fibrinogen to form fibrin. (3) In still higher concentrations (8 gm., or more, per 100 cc.) it delays, or completely inhibits, clotting by peptizing the fibrin and so preventing its orientation into the solid structure of the clot.