An association between the production of rheumatoid factor (RF)-like antibodies in animals immunized with bacterial immunoglobulin (Ig)G-binding proteins has been noted. Three potential explanations have been proposed: (1) altered host IgG due to binding of the immunogen; (2) B-cell superantigenic properties of the binding proteins; and (3) idiotype-anti-idiotype response leading to an antibody which acts as an antigen mimic. In order to distinguish among these possibilities, it is necessary to carry out studies in animals whose IgG does not react with the IgG-binding protein immunogen. Consequently, we have determined the effects of immunizing chickens with a purified group C streptococcal IgG-binding protein, FcRc, a bacterium expressing this protein, and appropriate control immunogens. The results of these studies provided evidence for production of specific antibodies to FcRc in groups of chickens immunized with either the pure protein or bacteria expressing the protein. No significant association with production of RF-like antibodies was noted, favouring the altered IgG-binding explanation for the association between RF-like antibodies and immunization with the bacterial IgG-binding proteins.