It was found that immobilized IgG on polymer carriers activates complement on contact with the serum. As polymers were microspherical in this study, complement fragments bound to polymers were detected by the agglutination of the polymer microspheres with the corresponding antisera or rosette formation with cells having complement receptors. Without the immobilization of IgG, polymers having amino, carboxyl, cyano or phenyl groups activated complement in the serum, while the presence of hydroxyl and carbamoyl groups in polymers did not cause complement activation. When intact IgG was bound to poly(glyceryl methacrylate) by the use of glutaraldehyde, the IgG-polymer conjugate activated complement in spite of the inertness of the polymer itself. The polymers immobilizing F(ab')2 activated complement less than the polymers immobilizing intact IgG. When dextran aldehyde prepared by periodate oxidation of dextran was used as a binder instead of glutaraldehyde, complement activation by F(ab')2-polymer conjugate was remarkably reduced, though antibody activity for binding the antigen remained. These results should be taken into consideration in the design of an immunosorption therapy.