In the application of the anti-Clq, the anti-C3, or the conglutinin-binding assay for the detection of complement-fixing circulation immune complexes in patients with ankylosing spondylitis or healthy blood donors, antibodies cross-reacting with the solid-phase bound agent were observed in several sera, resulting in erroneous interpretation of the tests. Corrections for the interfering antibodies were made by testing for the binding to conglutinin both in the presence and in the absence of Ca++, while the anti-Clq and the anti-C3 assays included the application of F(ab')2 fragments of normal IgG to check for anti-F(ab)2 anti-bodies.