Serum levels of C4, C3, IgG, IgM and IgA were measured by immunodiffusion in age matched groups of 48 sporadically encountered patients with ankylosing spondylitis, 56 with rheumatoid arthritis and 52 normal controls. Mean levels of C4 and IgA were significantly elevated in the sera of patients with sporadic ankylosing spondylitis, in comparison to normal controls. Mean IgA levels were also significantly elevated in 11 HL-A 27 positive patients with familial spondylitis but no such elevation was found in the sera of their healthy first-degree relatives with and without HL-A 27. The results of these studies suggest that humoral immune mechanisms may be involved in the pathophysiology of ankylosing spondylitis.