The C1q binding assay and the nephelometric monoclonal rheumatoid factor assay were able to discriminate 79% and 57% respectively of rheumatoid arthritis (RA) patients from healthy blood donors. In addition these assays could distinguish those patients with active arthritis from those with inactive disease, and the C1q binding assay correlated significantly with other laboratory indices of the rheumatoid process, including the erythrocyte sedimentation rate, low molecular weight or 7S IgM, and the rheumatoid factor titre. High levels of C1q binding were also seen in rheumatoid vasculitis. Both assays gave higher mean values in synovial fluid compared with the corresponding serum, but it appeared from ultracentrifugal analysis and from a lack of a consistent correlation between these assays that each assay was measuring different forms of immunecomplex-like material which may be involved in the immunopathogenesis of this disease. The C1q binding assay is of some value in the laboratory assessment of rheumatoid arthritis and appears to offer greater advantages than the monoclonal rheumatoid factor assay, although the usefulness of this latter assay may be very dependent on the monoclonal rheumatoid factor used.