A monoclonal antibody directed against the collagenous portion of human C1q was used to detect C1q-bearing immune complexes in patients with rheumatic disorders. Sera of patients with rheumatoid arthritis, systemic lupus erythematosus (SLE), osteoarthritis, as well as normal human sera (NHS) used as controls were tested in an ELISA system. C1q-bearing immune complexes were bound to a solid-phase monoclonal anti-C1q antibody, and detected with F(ab')2 antibodies to human IgG. Heat-aggregated human IgG was adjusted to the same concentration as the WHO standard for immune complexes and used for the standard curve in NHS. The mean value in NHS was 19.5 micrograms/ml equivalents of aggregated IgG. Using 2 SD over the mean as the upper limit for normal values, samples greater than 43 micrograms/ml were considered positive. Patients with osteoarthritis were negative; high levels of C1q-bearing immune complexes were detected in patients with rheumatoid arthritis (up to 800 micrograms/ml equivalents of aggregated IgG). With our assay C1q-bearing immune complexes were detected with high frequency (81%) in the sera of patients with rheumatoid arthritis, while a C1q solid-phase binding assay (C1q SPBA) revealed positive results only in 67% of rheumatoid arthritis sera. Compared to NHS, CH50 titers and C1q values of sera from patients with rheumatoid arthritis were frequently high. In contrast, the sera of SLE patients with low CH50 titers and low C1q levels had IgG immune complexes which could be detected only in the C1q-SPBA. C1q-bearing immune complexes were not detectable in the sera of patients with SLE. Since C1q triggers activation of the classical C pathway, this assay with monoclonal anti-C1q antibody appears to be useful for detecting immune complexes in rheumatoid arthritis patients with normal or elevated CH50 and C1q values, especially in the early stage of the disease.