Fibrinogen, isolated from canine plasma by the successive procedures of (1) freezing and thawing, (2) fractional precipitation with 25% saturated (HN4)2SO4 and (3) Sepharose 6B gel-filtration, had a molecular weight of 282 000 by the rapid sedimentation equilibrium method. However, a molecular weight for canine fibrinogen of 332 000, which is closer to that reported for human and bovine fibrinogens (340 000 plus or minus 20 000), was obtained from the sum of the molecular weights of the Aalpha, Bbeta and gamma chains, determined from dodecylsulfate gel electrophoretic patterns of reduced fibrinogen. Canine fibrinogen, subjected to proteolysis by urokinase-activated plasminogen for 24 h, contained degradation fragments D and E which were isolated by starch block electrophoresis and Sephadex G-200 gel-filtration. The purified D and E fragments with sedimentation coefficients of 5.0 S and 2.5 S had weight average molecular weights of 89 000 and 42 000, respectively by the rapid sedimentation equilibrium method. The ratio of D to E was 2:1 per parent fibrinogen molecule. Antigenic analysis according to anti-fibrinogen antiserum showed that both D and E fragments were antigenically deficient to native fibrinogen and revealed a reaction of non-identity with each other. Upon immunoelectrophoresis at pH 8.2, D and E had different electrophoretic mobilities. Preliminary studies indicate that based on thrombin time alone, D has anticoagulant activity while E appears to be a coagulation potentiator. Canine fibrinogen apparently consist of two core fragments with dissimilar chemical characteristics in common with the fundamental structures of human and bovine fibrinogens.