Lymphocytes from HLA-B27 positive patients with ankylosing spondylitis or Reiter's syndrome and from matched controls were cultured with pokeweed mitogen and heat-killed S. aureus bacteria under conditions designed to maximize immunoglobulin production. Despite the secretion of microgram quantities of immunoglobulin, which were not significantly different in patients and controls, only negligible amounts of IgM, IgG and IgA antibodies to Campylobacter, Shigella and Yersinia were detectable. In addition, no anti-HLA-B27 antibodies were produced by any patient. Our results are not consistent with the hypothesis that HLA-B27 crossreactive antibodies to Enterobacteriaceae antigens have a role in the pathogenesis of ankylosing spondylitis or Reiter's syndrome.