Previous work has shown that the intermediate complexes isolated from the plasma of patients with rheumatoid arthritis are composed of self-associating IgG rheumatoid factors. Mice of the MRL/1 strain develop spontaneous autoimmune disease with arthritis that is pathologically similar to human rheumatoid arthritis. Also, the sera of MRL/1 mice contain autoantibodies to nuclear antigens as well as IgM and IgG rheumatoid factors. The present studies were done to determine if the IgG rheumatoid factors isolated from these mice undergo self-association. MRL/1 mouse sera were categorized into groups A and B based on serumserum precipitin interactions. Thirteen of 13 MRL/1 mice sera examined contained intermediate complexes sedimenting between the 6.6S and 19S components of normal serum by sedimentation velocity ultracentrifugation. There were no differences in the level of intermediate complexes between groups A and B. IgG rheumatoid factors were isolated from the sera of 9 other mice. Upon sedimentation equilibrium ultracentrifugation, these rheumatoid factors underwent concentration-dependent self-association similar to that described for human self-associating IgG rheumatoid factors, although the precise stoichiometry of self-association could not be determined. The IgG rheumatoid factors from group B had higher energies of self-interaction than those from group A. These studies provide additional evidence that MRL/1 mice may be the best available animal model for the study of human rheumatoid arthritis.