Serological characterization of fibrinogen and its derivatives was attempted by using antibody-coated latex agglutination and counterelectrophoresis. It was found that: 1) fibrinogen was not only capable of agglutinating uncoated latex particles but also fixed to the latex particles; 2) the sensitivity of some of the latex reagents used for the detection of fibrinogen may reflect the antibody activity to the derivatives with altered antigenicity in which FDP-E antigen is more readily available than in intact fibrinogen molecules, as the one contained in Liquid Human Plasma but absent in its batches prepared in the presence of plasmin inhibitors; 3) increase in FDP-E antigenicity was noticed in the course of fibrinogenolysis so its availability would indicate the occurrence and extent of proteolytic process on fibrinogen; and 4) FDP-E-antigen developed in the course of plasmin digestion may not be shared by intact fibrinogen since the former was reactive in counterelectrophoresis against anti-FDP-E serum even when the latter had been absorbed with fibrinogen.