This study investigated the occurrence of an autoantibody, IgM rheumatoid factor, that may result from the chronic inflammation noted in periodontal disease and rheumatoid arthritis. In order to detect IgM-RF, a biotin-avidin ELISA was developed. This assay was found to be sensitive and accurate by testing a rheumatoid arthritis population. The characteristics of this rheumatoid arthritis group were further determined, such that the total serum immunoglobulin concentrations were slightly elevated although within the normal range for IgM, IgG, and IgA; IgG antibody levels were elevated against oral microorganisms of the genus Capnocytophaga, while elevated IgM antibody levels were noted to Bacteroides species. In a population of 260 subjects of which 171 were periodontal disease patients, 16 of 171 (9.4%) were seropositive for IgM-RF, of which the predominant disease types were advanced destructive periodontitis and adult periodontitis. For comparison, a random population of seronegative periodontal disease patients was constructed that was matched for sex and approximate age to the seropositive group. The total immunoglobulin levels of the two groups were not significantly different and the means of both were slightly lower than the rheumatoid arthritis group. When the antibody profiles of the two periodontal disease populations were compared it became evident that the RF-positive group showed IgM and IgG antibody that was significantly elevated to Capnocytophaga species and F. nucleatum. Therefore, the chronic inflammation associated with periodontitis appears to increase significantly the formation of IgM-RF; however, there does appear to be a relationship between IgM-RF and elevated antibody to selected oral microorganisms.