We developed a microtiter solid-phase radioimmunoassay for quantitating C3 breakdown products (iC3b, C3dg, C3d) in human plasma with a unique monoclonal antibody specific for a neoantigen present on iC3b and C3d (MoAb 130). This monoclonal antibody reacts with a neoantigen which appears when C3b is converted to iC3b. The neoantigen is also present on the C3dg and C3d fragments derived from iC3b. The concentration of the neoantigen is elevated in the plasma of most patients with rheumatoid arthritis and systemic lupus erythematosus as compared to normal volunteers. Some patients with glomerulonephritis also had elevated concentration of the neoantigen in their plasma.