Abstract A simple method for rapid determination of IgG-containing circulating immune complexes by commercially available reagents was developed. In this method, serum is incubated with 2.5% polyethyleneglycol. After washing, the precipitate is incubated with radioactively labeled protein A, which binds to IgG in the immune complex. After a further washing, the radioactivity bound is measured. Artificially formed complexes of heat-aggregated human IgG are diluted and a reference curve is constructed. This method is compared to the solid phase C 1q-binding method. Our method is slightly more sensitive than the C 1q-binding method.