Abstract: An immunoadsorbent material for removing IgG and IgG-complexes from biological fluids is prepared by covalently binding protein A to a solid-phase silica matrix. It has been found that particularly stable, high-capacity immunoadsorbents are obtained by derivatizing the silica with amino and/or carboxyl groups, and reacting the protein A with a carbodiimide at a pH in the range from 3.5 to 4.5. Binding through free hydroxyl groups may be achieved with cyanogen halides at a pH in the range from 11.0 to 11.5. After acid washing (pH 2.0-2.5) to remove non-covalently bound protein A, the immunoadsorbent may be employed in a column for therapeutic treatment of various cancers and autoimmune disorders where IgG-complexes are implicated as suppressing factors in inhibiting a normal immune response.