Chromatographed thrombin in the presence of both 50 Kallikrein inhibitor units of Trasylol per ml and 0.1 M E-ACA solubilized fibrin and the products of lysis possessed anticoagulant properties. The peak of the antithrombic activity coincided with the time of complete lysis of the fibrin clot, plasmin lysed fibrin exhibited the peak of its antithrombic activity much earlier. The effect of thrombin lysed fibrin on the prothrombin consumption of shed blood was found to be inhibitory. The products of the digestion of fibrin by thrombin and by plasmin, isolated at an advanced stage of proteolysis were compared by gel filtration, disc electrophoresis and DEAE cellulose chromatography. Differences in physical characteristics of these fibrin breakdown products offer evidence that they were produced by two different enzymes. * Presented in part at the 53rd Annual Meeting of FASEB, Atlantic City, 1969 and at the 17th Annual Symposium on Blood, Wayne State University School of Medicine, Detroit, 1969. Dr. Muirhead’s present address is Ayerst Laboratories, 1025 Laurentian Blvd., St. Laurent, Quebec, Canada. Dr. Triantaphyllopoulos’ present address is American National Red Cross, Blood Research Laboratory, 9312 Old Georgetown Road, Bethesda, Maryland 20014. Reprints should be forwarded to Dr. C. Muirhead.