The present invention is concerned with the radioimmunoassay (RIA) procedure for assaying body fluid content of an antigenic substance which may either be an antigen itself or a hapten capable of being converted, such as by means of reaction with a protein, to an antigenic material. The present invention is concerned with a novel and improved modification of a double-antibody RIA technique in which there is a first antibody that is specific to the antigenic substance suspected to be present in a body fluid from which the assay is intended. The second antibody, however, is not specific to the antigenic substance or analyte, but is an antibody against the first antibody.PPIn accordance with the present invention, the second antibody may be covalently bonded to a water-insoluble organic polymeric substance, such as cellulose or other polysaccharide, a vinyl addition polymer or condensation polymer, such as an aminoplast or a polyester, or a water- insoluble inorganic substance of polymeric nature, such as glass or silicone resins or the second antibody may be adsorbed to the surface of a solid support, such as polystyrene or polypropylene. The antigenic substance, that is, the analyte to be assayed, hereinafter sometimes referred to broadly as a "ligand", may be a protein or a polypeptide, or it may be a vitamin, a drug, a glandular extract or secretion, such as a hormone of steroid or other type.
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