Molecular sizes of fibrinogen (F) similar to FI, higher molecular weight form, and FII, lower molecular weight form, have been found by Lipinska and colleagues. A procedure has been developed to isolate for the first time each of the FI and FII forms of fibrinogen which are free of each other and of high molecular weight fibrin-fibrinogen complexes. This process involved removing the complexes by A-5 m chromatography. This chromatography also reduced a protein contaminant (X) and removed plasminogen. (NH4)2SO4 subfractionation at pH 5.9 was then done. A subfraction (16-18%) containing 90% FI and another (22-25% or 25-28%) containing 96% FII were obtained. Reprecipitation of the first 16-18% subfraction yielded a subfraction containing 97% FI. sodium dodecyl sulfate (SDS) - polyacrylamide gel electrophoresis of FII revealed that it contains one intact a alpha chain per (b beta, gamma)2. Clot opacity studies on FII suggested that the carboxyl terminal portion of the alpha chain of fibrin plays an important role in the lateral associations in fibrin polymerization. Also, the pattern of (NH4)2SO4 precipitation of the endogenous fibrin-fibrinogen complexes was studied. This revealed that the complexes precipitated mostly in the least soluble subfractions, but small amounts could be found in all subfractions. Examination of the complexes by SDS-polyacrylamide gel electrophoresis showed that most of the complexes could be dissociated to FI and FII. However, there were complexes which remained and these were found to be covalently cross-linked forms probably produced by factor XIII.