Immune complexes were measured in the sera of 18 patients iwth ankylosing spondylitis and 5 normal control subjects by their ability to inhibit antibody-mediated lymphocyte-induced cytotoxicity. 11 of the spondylitis and none of the control sera contained circulating complexes. Fractionation of sera on Sephadex G-200 showed that complexes were of two molecular sizes, one of intermediate and the other of large molecular weight. The presence of immune complexes did not correlate with activity of disease or the presence of peripheral arthritis although the one patient with polyarthritis had the highest level of complexes detected in this study. The nature of the antigen in these antigen-antibody complexes is of great interest since it may provide evidence of the aetiological agent in the disease.