In order to investigate the relationship between renal histopathology and the characteristics of circulating immune complexes (CICs) in patients with lupus nephritis (LN), we measured the sizes of CICs, DNA-bound immunoglobulins in patients with systemic lupus erythematosus (SLE) and different histopathological forms of nephritis. Sera were obtained from nine patients: four with diffuse proliferative LN (DPLN), four with membranous LN (MLN), and one with mesangial LN, who fulfilled the criteria of the American Rheumatism Association for SLE. The DNA-bound immunoglobulins were measured by ELISA, in which ELISA plates were coated with mouse monoclonal anti-DNA antibodies. The sizes of CICs were analysed by sucrose density gradient ultracentrifugation. Large (larger than 19S), intermediate (19-7S) and small (nearly 7S) sized DNA-bound immunoglobulins (high peaks of IgG and IgA, but low IgM peaks) were found in the patients with DPLN. By contrast, in patients with MLN, the sizes of ICs; DNA-bound IgG, IgA were in general slightly larger than 7S. In one patient with DPLN, at the onset, various sized DNA-bound IgG, IgA and IgM were found. After the methylprednisolone pulse therapy, CICs became smaller and gradually disappeared. We conclude that the characteristics of DNA-anti-DNA IgG, IgA complexes may determine the localization of ICs in the glomeruli and suggest that CICs play an important role in the pathogenesis of LN.