Readily demonstrable IgG complexes are a characteristic feature of the serum and joint fluid of most patients with rheumatoid arthritis. The complexes that occur in the joint fluids have been implicated in the activation of the complement system. Studies with highly purified radiolabeled joint fluid complexes, dissociated in acid buffers, showed that isolated 7S fractions contain all the components necessary to readily reassociate and reform rapidly sedimenting complexes. Furthermore, quantitation of anti-IgG activity in the dissociated 7S fraction demonstrated that the preponderance of molecules possessed anti-IgG activity. These findings provided evidence that IgG anti-IgG molecules account for the major proportion of the precipitable IgG complexes. Quantitation by analytic ultracentrifugation revealed considerable amounts of complexes not detected by precipitation with the monoclonal anti-IgG reagents, which thus emphasizes the existence of a spectrum of IgG complexes with differing physical and biologic properties. Sodium sulfate, used at varying concentrations, proved to be useful for differentially enriching various fractions of the complexes.