Abstract: An improved radioimmunoassey procedure involves the use of an improved reusable immunoadsorbent through which a sample of an antigen and radioactive labelled antigen is flowed to form a bound and unbound fraction. The bound fraction is releasably coupled to the immunoadsorbent which includes a solid particulate substrate to which antibodies, specific to the antigen, are covalently bonded by reaction between the carboxyl or amine group of the antibody and a group such as an acyl azide, carbonate, thiocarbonate, polythiol, isocyanate, epoxide and chlorothioformate group, for example, reacted on the substrate. The substrate includes a porous refractory material having at least one polymeric material bonded thereto as a barrier and to which is linked one of the above groups for reaction with the carboxyl or amine group of the antibody. The antigen bound to the immunoadsorbent is stoichiometrically released by an eluting solvent and the immunoadsorbent is reused.