The concentration of IgA and titre of IgA antibodies to several Gram-negative bacteria were measured in the serum and parotid saliva of patients with AS and normal tissue-typed individuals. Salivary IgA and antibody levels in the patients were identical with the control population. The serum antibody level against Yersinia enterocolica 0:3 was slightly raised in patients but there was no difference in the reactions to Klebsiella oxytoca strain MX100 or Escherichia coli 0111.B4. The serum IgA level was elevated in patients with AS, irrespective of HLA B27. We conclude that this approach is unlikely to provide convincing evidence of a link between Gram-negative bacteria and ankylosing spondylitis.