To find out whether disease activity and B27 status were associated with serum concentrations of IgA, C reactive protein (CRP), and haptoglobin in ankylosing spondylitis (AS) multivariate analysis of variance was used to study 101 patients with AS whose disease was clinically classified as active or inactive, and who were HLA-B27 typed. It was found that B27 and disease activity do interact significantly to affect the serum concentrations of IgA, CRP, and haptoglobin. When the 77 B27+ patients were examined, however, it was found that disease activity was significantly associated with serum concentrations IgA. In contrast, in the 24 B27- patients concentrations of serum IgA were significantly associated with disease activity, but not. These results emphasise the known difference between B27+ and B27- AS and suggest different pathogenic mechanisms in the two forms of AS.