The presence of immunoglobulin is shown to alter the network structure of gels formed from purified fibrinogen. Turbidity measurements were used to calculate the effect of increasing concentrations of immunoglobulin on the mass-length ratio of fibrin fibers composing the gels. Normal pooled, human IgG was found to reduce the mass-length ratio of fibrin fibers composing the gel from 7 X 10(12) daltons/cm with no immunoglobulin to 1 X 10(12) in the presence of 40 mg/ml. Monoclonal IgGk was found to be more effective in the reduction of the mass-length ratio than normal IgG. This observation indicates that the immunoglobulin must decrease lateral association of fibrin protofibrils. As the mass-length ratio decreased, the rigidity of the gels increased as the immunoglobulin concentration was elevated (1.2 dynes/cm2 with no immunoglobulin to 2.9 at 15 mg/ml). Since the fibrin fiber diameters were decreased, the number of interfiber crosslinks must have been increased to explain the increase in rigidity. Antifibrinogen, antifibrin intermediate, and antithrombin antibody activity of the immunoglobulins were proved unlikely.