In the resolution of fibrin clots the liquefied fibrin is found to yield unaltered prothrombin and a new protein, coagulating at 65[degree] C, which probably represents the fibrinogen in an altered form. No unchanged fibrinogen or active thrombin are found in the soln. Prothrombin can be used time after time in fibrin formation, and be recovered with little loss after resolution of the clot. It always requires new Ca and cephalin, however, for its activation to thrombin.