Abstract Serum IgG anti-IgG antibody was measured in patients with rheumatoid arthritis and osteo-arthritis and in healthy young adults. The antibody was isolated by immunoadsorption and evaluated by titration with latex particles. After establishing the method's specificity for IgG anti-IgG, its reproducibility and a means for converting titer to IgG concentration, IgG anti-IgG was found in greatest amounts in rheumatoid arthritis when compared with two other groups. The concentration of IgG anti-IgG, however, was consistently lower in our study than in studies by other workers. This discrepancy appears to be related to our procedure, namely dilution of serum prior to exposure to immunoadsorption, the latter tending to dissociate immune complexes (removal of IgG antigen from IgG anti-IgG antibody). The eluate obtained in this manner contains antibody IgG freed of IgG antigen, as manifested by mainly 7 S IgG, rather than 7 S IgG admixed with complexes of higher sedimentation coefficients. The method reported in this article, therefore, appears to measure IgG anti-IgG antibody with a greater degree of accuracy than previously reported techniques.