Abstract Recent studies have provided much new information on the structure of the plasma protein fibrinogen. N-terminal analyses of fibrinogen and fibrin (Blombäck and Yamashina, 1958) have suggested that the native molecule is a dimer consisting of three pairs of polypeptide chains. This idea is supported by studies on the conversion of fibrinogen to fibrin by thrombin which have shown that two A peptides and two B peptides are released per fibrinogen molecule ( Blombäck, 1958; Gladner et al. , 1959 ). This report described two methods for the dissociation of fibrinogen and a partial characterization of the products by physical means. The results support the model given above and suggest that the polypeptide chains of fibrinogen are linked by disulfide bridges.