The specific methylation of fibrinogen with dimethylsulfate resulted in modification of aspartic and glutamic acid groups which was ascertained by chromatographic analysis of the products of an enzymatic digest of the modified fibrinogen. The trimethyloxonium ion-modified fibrinogen produced an ultrastructure of the methylated fibrinogen which resembled the thrombin-induced physiological clot formation. Evidence of periodicity in the electron micrographs of the methylated fibronogen was achieved by a low incorporation of methyl groups into the fibrinogen. High levels of modification produced a polymerization that was less ordered structurally. The proteolytic digestion of fibrinogen by plasmin is considerably altered by citraconylation, as was demonstrated by disc electrophoretic petterns. Fragments D and E usually generated from native fibrinogen were not present; however, higher molecular weight components were obtained. This is similar to observations made on patients with certain abnormal fibrinogens. Saponification of the methylated fibrinogen polymer resulted in solubilization and regenerated native fibrinogen. The action of thrombin on the demethylated fibrinogen produced a fibrin clot with the release of peptides A and B at physiological rates.