Anti-IgG antibodies were detected in 69% of 235 sera tested. A simple and reproducible method was developed to measure the avidity index of anti-IgG antibodies. It was found that subjects with systemic lupus erythematosus and progressive systemic sclerosis and normal controls had antibodies with lower avidity index than those with rheumatoid arthritis and Down's syndrome. The difference in avidity seems to be disease specific. Avidity does not correlate with antibody titers, but all sera with high avidity antibody are positive in the latex test. The anti-IgG as measured by this method was shown to be mainly of IgM class. The avidity indices of whole sera and purified IgM fractions were similar.