OBJECTIVE: To study antibodies to Escherichia coli O:14, which expresses large amounts of enterobacterial common antigen (ECA), and their corresponding antigen molecules in serum and synovial fluid samples from patients with rheumatoid arthritis (RA). METHODS: Enzyme linked immunosorbent assay (ELISA) was used to measure antibodies to heat killed E coli O:14 in serum and synovial fluid samples from patients with RA and control subjects including healthy donors and patients with osteoarthritis. ELISA was also used to perform absorption analyses of antibodies to E coli O:14 with several enteric bacteria and their lipopolysaccharide (LPS). In addition, antigenic molecules reacting with E coli O:14 antibodies from patients with RA were examined using immunoblot analysis and N-terminal amino acid analysis. RESULTS: Compared with control subjects, patients with RA showed significantly increased titres of antibodies against heat killed E coli O:14 in 33 of 83 serum samples (39.8%) and 38 of 58 joint fluid samples (65.5%). Absorption analyses with enteric bacteria and their LPS resulted in the reduction of antibody titres to heat killed E coli O:14 in serum and synovial fluid samples from the RA patients. In addition, immunoblot analysis of the samples from RA patients revealed not only a ladder-like banding pattern equivalent to ECA associated with LPS, but also two clear bands of bacterial outer membrane proteins of 35 kDa (Omp A) and 38 kDa (Omp C), having amino acid sequence homology with those of other Enterobacteriaceae. CONCLUSION: These results suggest that some patients with RA are sensitised to antigens common to Enterobacteriaceae, and this may prove relevant to the future development of immunotherapy for RA. Furthermore, this sensitisation to antigens found commonly in Enterobacteriaceae may have a key role in the pathogenesis of human RA similar to that described previously in our animal model.