Abstract: Solid supports for solid phase immunoassays comprise polyethyleneimine-coated negatively charged polymeric materials. The combination of negatively-charged polymer support and polyethyleneimine coating serves to eliminate nonspecific adsorption of biological molecules such as high molecular weight kininogen, which have affinity for solid surfaces and which interfere with immunoassays. The negatively charged polymeric support to which the polyethyleneimine coating is applied may advantageously comprise, for example, a carboxylate-modified styrene microparticle. Antigens or antibodies are coupled by covalent coupling agents to the polyethyleneimine coating to form solid phase immunoreagents. Immunoassays for biological molecules such as high and low molecular weight kininogen are provided.