Serum levels of immunoglobulin A (IgA) and the complex immunoglobulin A-alpha 1 antitrypsin (IgA-alpha 1AT) were measured at the commencement and after 3 months of a double-blind, placebo-controlled trial of sulphasalazine (SAS) in patients with active ankylosing spondylitis (AS). Twenty-eight patients were evaluated, 15 on sulphasalazine, 13 on placebo. Significant falls were seen in both IgA (p less than 0.01) and IgA-alpha 1AT (p less than 0.001) in the actively treated patients. In addition, significant improvement in clinical and laboratory measures of disease were observed. It is concluded that SAS is effective in AS and modulates the immune response.