A congenital dysfibrinogen characterized by impaired fibrin monomer polymerization was found in an asymptomatic 50-year-old woman and her two sons. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) run according to the method of Laemmli, we noticed two gamma chain species in fibrinogen and its plasmic fragments D1 and D2, consisting of a normal species and an apparently lower molecular weight (mol wt) variant in respective fractions. However, in fragment D3 only a single gamma chain remnant was observed. By chromatofocusing of the plasmic-CaCl2 digests of the abnormal fibrinogen, we separately isolated the normal and abnormal D1 species, the latter having been eluted in a slightly higher pH range. As expected, the abnormal D1 species failed to interfere with thrombin clotting of normal fibrinogen and normal fibrin monomer polymerization, whereas the normal D1 species inhibited them markedly. By analyzing the lysyl endopeptidase digests of the isolated gamma chain, we identified a replacement of aspartic acid by tyrosine at position 330 of the mutant gamma chain. The point mutation from an aspartic acid (pK for the beta-carboxyl = 3.86) to a tyrosine (pK for the aromatic hydroxyl = 10.07) may have perturbed the folding gamma chain structure in the D domain of fibrinogen specifically required for polymerization.