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In serum samples from 37 patients with ankylosing spondylitis (AS), immune complexes were quantitated by the 125I-Clq binding test; in paired plasma samples the C3 breakdown product C3d was measured by an immunochemical method. Compared to results in 30 blood donors, the Clq binding activity was significantly (greater than 2 SD), although discretely, increased in 5 of 8 patients with seropositive AS, but not in other AS patients. All C3d levels were within the normal range. In a parallel investigation, increased Clq binding activity and C3d levels were found in 87% and 90%, respectively, of patients with seropositive rheumatoid arthritis.