γG globulin complexed in an unusual form has been demonstrated in the serum of many patients with rheumatoid arthritis. Such complexes have been detected and isolated principally through precipitation reactions with monoclonal γM rheumatoid factors. These monoclonal rheumatoid factors exhibited a greater sensitivity to react with small complexes or aggregates of γ-globulin than polyclonal rheumatoid factor from rheumatoid arthritis sera or isolated C1q. The serum complexes consisted in large part of high molecular weight but acid-dissociable 7S γG globulin molecules They however differed from the complexes in the joint fluid by not yielding precipitates with C1q and were not found in association with evidence of marked serum complement fixation or activation. A small number of systemic lupus erythematosus sera, primarily those forming cryoprecipitates, also gave reactions with monoclonal rheumatoid factor. Sera from patients with a variety of nonrheumatic diseases gave a low incidence of reactions. The exact nature of the complexes in the rheumatoid arthritis sera remains somewhat in doubt although γG rheumatoid factors appear partly involved.