Conversion of fibrinogen to fibrin polymer has been investigated. It is noteworthy to find that excess basic amino acids, either L-arginine or L lysine, are capable of polymerizing fibrinogen at neutral pH simply by charge-neutralization process without removing fibrinopeptides. This observation strengthens the notion that both specific orientation and balance of electric charge of the protein molecule are essential factors for the fibrin polymerization. Chemical modification of arginine residue(s) of fibrinogen completely inhibits thrombin-catalyzed conversion to fibrin polymer, which confirms that the specific arginine is recognized as a cleavage site by thrombin to yield fibrinopeptides and fibrin. The molecular weights of three different polypeptide chains of fibrinogen are reduced to lower than 47,000 upon proteolytic digestion with chymotrypsin. Surprisingly, however, these lower molecular species of fibrinogen are found to be polymerized in the presence of thrombin. Therefore, extensive structural analyses of the chymotrypsin-treated fibrinogen will contribute to define the essential interaction domains of fibrin units required for polymerization.