A previously undescribed gamma chain variant of human fibrinogen has been identified by application of a sensitive sodium dodecyl sulfate-polyacrylamide gradient gel electrophoretic technique to separate the polypeptide chains. This variant, called gamma B, clots and cross-links as well as the major species (gamma A), is similarly degraded by plasmin, and has a molecular weight of 53,100 as compared to 50,100 for gamma A. Cross-linked dimers of Mr = 100,100 and 108,500 are identified after action of thrombin and Factor XIIIa, suggesting the formation of gamma A-gamma A and gamma B-gamma B dimers rather than gamma A-gamma B hybrid species. The gamma B chain dimers represent 16% of the total cross-linked gamma chain forms. Two early plasmic derivatives of gamma A and gamma B chains have been demonstrated to have lost fragments of Mr = approximately 4,000 and 5,000 without loss of 5-dimethylaminonaphthalene-1-sulfonyl cadaverine fluorescence. Since the difference in molecular weight of gamma A and gamma B chains is maintained during plasmic degradation of both monomer and dimeric fluorescent forms, this suggests that the additional sequence of amino acids in gamma B is located at or near the COOH-terminal end of this polypeptide chain.