Several forms of seronegative polyarthritis are strongly associated with HLA-B27, and a number of microorganisms have been implicated in the etiology of these diseases. To explain the association between HLA-B27 and arthritis initiated by infection with these organisms, it has been proposed that there is immunologic cross-reactivity between the B27 molecule and 1 or more microbial antigens, and that this cross-reactivity leads to tolerance to such infection and/or to the production of anti-HLA-B27 cross-reactive antibodies. Such cross-reactive antibodies were detected in the sera of only 2 of 63 patients recently infected with Shigella flexneri, Campylobacter jejuni, or Yersinia enterocolitica who had highly significant antibody levels against the infecting bacterial species. Most striking was the absence of anti-HLA-B27 antibody in the serum of 16 of 17 patients who developed reactive arthritis following Yersinia infection.