The cryofibrinogen of 27 patients was studied by SDS-polyacrylamide gel electrophoresis and immunochemical methods. Electrophoretic analysis of the isolated cryofibrinogens, as well as the proteins left after heat or thrombin defibrination, showed that cryofibrinogen is composed of two proteins, fibrin(ogen) and cold-insoluble globulin (CIg). A proportion of the fibrin(ogen) component formed stabilized oligomers interlinked through γ-γ dimerization. The degree of fibrin(ogen) proteolysis, as judged by measuring the α: γ ratio of the reduced samples, was very similar to that of the fibrinogen of the original plasma. The CIg:fibrin(ogen) molar ratio in the cryofibrinogens was 0.041 ± 0.018. The CIg and the fibrin(ogen) content of the cryofibrinogens were strongly correlated with the plasma CIg le