Undifferentiated oligoarthritis (UOA) resembles clinically reactive arthritis (ReA), but does not fulfill the diagnostic criteria. In 46 patients with UOA, in 16 with ReA, and in 15 with rheumatoid arthritis (RA) the humoral and cellular immune response to the ReA-associated bacteria Chlamydia trachomatis, Yersinia enterocolitica, Shigella flexneri, Salmonella enteritidis, Campylobacter jejuni and Borrelia burgdorferi was investigated in paired samples of synovial fluid and peripheral blood. An antigen-specific lymphocyte proliferation in SF was found in 75% of the ReA and in 39% of UOA patients, but in none with RA. Shigella and Chlamydia in ReA and Yersinia and Chlamydia in UOA were the most frequent stimulating antigens. There was a poor correlation between antigen specific lymphocyte proliferation and specific antibodies. We conclude that these bacteria might have a pathogenetic role, not only in ReA, but also in UOA.