OBJECTIVE To further evaluate the role of bacterial antigens in triggering inflammation in the joint in patients with reactive arthritis by studying local antibody synthesis in the joint. METHODS Yersinia-specific antibodies in paired serum and synovial fluid samples from 29 patients with yersinia triggered reactive arthritis were studied using an enzyme linked immunosorbent assay (ELISA), an inhibition ELISA with six monoclonal antibodies against lipopolysaccharide or released proteins of yersinia and immunoblotting. Antibodies of IgM, IgG and IgA classes, as well as antibodies of IgA subclasses and those containing secretory component were measured against the lipopolysaccharide and the sodium dodecyl sulphate extract of whole Yersinia enterocolitica O:3 bacteria. RESULTS It was shown that yersinia-specific antibodies, as well as antibodies against other microbial antigens (rubella, measles, Bordetella pertussis, tetanus toxoid and Candida albicans) in synovial fluid mirror those in serum by concentration, by specificity and by distribution in classes and subclasses. CONCLUSIONS These results do not suggest any strong local antibody production, but indicate that the majority of yersinia antibodies in the synovial fluid are derived from the circulation.