Complement-fixing material in ther sera of patients with rheumatoid arthritis.

We investigated the properties of the complement-fixing material in samples of sera from fifteen patients with rheumatoid arthritis having low levels of C4, in order to determine its possible relationship to the immune complexes and rheumatoid factors. The complement-fixing material in most sera was of high molecular weight, 19s or greater, but in two sera it was also present in an intermediate zone (about 12s--15s). Cryoprecipitation did not remove the complement-fixing material except from serum with a elevated content of cryoglobulin. Analysis of the sera by analytical ultracentrifugation for the content of rheumatoid factor IgG complexes did not show a close relationship between the content of these complexes and the amount of complement-fixing material present in the sera. The responsible material could be absorbed by IgG-Sepharose together with the removal of rheumatoid factor. It was, however, possible to absorb the complement-fixing material from most sera with unconjugated Sepharose without significantly affecting the rheumatoid factor titres. Thus the complement-fixing material in the rheumatoid sera closely resembles an immune complex. It very likely contains rheumatoid factor, but it is not certain whether the material consists entirely of rheumatoid factors complexed to IgG or whether other antigen-antibody systems may also be present.