Twenty-two patients with active ankylosing spondylitis were investigated to assess the levels of specific serum IgG, IgA and IgM titres against Campylobacter jejuni/coli before and during treatment with sulfasalazine. An enzyme-linked immunosorbent assay was used, and the results were compared with the antibody levels in 300 healthy blood donors. Three patients had elevated levels of serum anti-Campylobacter-IgA before treatment, and a two-fold decrease in the antibody titre was observed during treatment. Three patients had elevated anti-Campylobacter-IgG titres before treatment. One of these patients also had elevated anti-Campylobacter-IgA and IgM titres. Elevated IgM titres were not seen in any other patient. The results do not support the hypothesis that C. jejuni/coli plays an important role in the pathogenesis of active AS.