The binding of IgM from a rheumatoid factor (RF-IgM) to IgG from 12 animal species was analyzed by an ELISA system. The RF-IgM bound various animal IgG with dissimilar affinities. The binding of RF-IgM to animal IgG was inhibited by addition of protein A, which binds some animal IgG by recognizing the junctional site on CH2-CH3 domains in the Fc region. As previously reported, no significant correlation was observed between the binding of RF-IgM to IgG and the content of galactose-free oligosaccharides, which is increased in IgG of rheumatoid arthritis patients or autoimmune mice. We suggest that the crucial epitope of IgG for RF-IgM binding is not the oligosaccharide structure generated specifically in IgG of autoimmune diseases but that RF-IgM may recognize a certain protein conformation of a region in IgG near the binding site of protein A.