A competitive protein binding method is provided for the determination of an analyte which is a member of an immunological pair consisting of ligand and receptor for the ligand. A chemiluminescent source is employed comprised of one or more individual members, one chemiluminescent source member being conjugated to one of the members of the immunological pair, so as to provide chemiluminescence adjacent to the site of conjugation. A quencher molecule is conjugated to a member of the immunological pair. When the members of the immunological pair bind, the quencher molecule is brought within quenching distance of the chemiluminescent source so as to inhibit the emission of light by the chemiluminescent source. The amount of analyte present in the assay medium affects the amount of binding between the members of the immunological pair which results in quenching of the chemiluminescence. By observing the light emitted from the assay medium, either from the chemiluminescent source of the quencher, the change in light emission in relation to the concentration of analyte present in the assay medium can be used to determine the amount of analyte present in the assay medium. By employing standards having known amounts of analyte, the amount of analyte in an unknown sample can be quantitatively determined.Reagent kits can be provided having predetermined amounts of the reagents, so as to substantially optimize the sensitivity of the assay.