Nylon microfiltration membranes were coated with dextrans (M r β 6000 and 40,000) and poly(vinylalcohol) (PVA) (M r β72,000) to produce affinity matrices with adequate chemical functionality and low non-specific protein binding characteristics. In all cases, a considerable reduction of non-specific adsorption was attained, with PVA being most effective. Also, the permeability was reduced, indicating the presence of coating polymers in the flow-through pores. The extent of permeability reduction was related both to differences in the molar mass of dextran, as well as to polymer characteristics. The highest reduction was observed with the coil-forming high-molecular weight dextran. Protein A was immobilized to the different membranes for the adsorption of human immunoglobulin G (IgG). The highest apparent association constants were obtained with PVA-coated membranes, followed by membranes which were coated with low-molecular weight dextran, and then by membranes, which were coated twice with high-molecular weight dextran. It is concluded that multiple covalent binding of protein A occurs in the open network of dextran coils, restricting the accessibility of the various binding sites of this ligand for the large IgG molecule, and therefore, weakening the ligand-ligate association. With the film-forming PVA, a better accessibility was obtained with comparable ligand density to commercial protein A chromatography supports.