In order to explore the alteration in immunological function in patients with ankylosing spondylitis (AS), serum concentrations of IgG, IgA, IgM, C3 and C4, and lymphocyte subpopulations defined by monoclonal antibodies (OKT series) were studied in 40 AS and 20 age-matched normals. The results showed that i) The IgG and IgA in AS patients were much higher than those in normals (p less than 0.001) and C3, C4 and IgM were also increased (p less than 0.01, less than 0.05, less than 0.05, respectively), ii) The percentage of B cells and OKT3 cells of AS patients were lower, but OKT8 and OKIa1 were higher than that of normals (p less than 0.05). When the AS patients were further divided into active group (22 cases) with c-reaction protein (CRP) higher than, or equal to 15 mg/l and inactive group (18 cases) with CRP less than 15 mg/l, it was found that the inactive group had decreased B cells (p less than 0.01) and OKT3 (p less than 0.05) as compared with normals. On the other hand, the active group had increased percentage of OKT8 than normal controls (p less than 0.05), and increased OKIa1 than both inactive group and normal controls (p less than 0.01), but OKT3 decreased than the normal (p less than 0.05). It is therefore concluded that the immunological aberration plays a role in the pathogenesis of AS.