The sera of patient with adult rheumatoid arthritis (RA), juvenile rheumatoid arthritis (JRA), systemic lupus erythematosus (SLE) and mixed connective tissue disease (MCTD) were employed together with sensitive radioimmunoassays to examine the relationship between immune complexes detected by three methods and the IgM, IgA and IgG classes of rheumatoid factors (RF). Compared to the controls, significantly increased concentrations of RF and immune complexes were detected for every patient group except JRA. Increased concentrations of IgA RF, IgG RF and immune complexes detected by the C1qBA and the C1qSP were strongly associated with the presence of IgM RF. Significant correlations were noted between RF (especially IgG and IgM) and immune complexes (detected by both the C1qBA and the C1qSP) for patients with RA, SLE and MCTD. These studies suggest that RF may be partially involved in the formation of the immune complexes detected by C1q-binding assays and demonstrate the need for further clarification of the constituents of the immune complexes detected in each disease.