Abstract: The present invention provides a method capable of simultaneous processing of plural test samples for the receptor binding property of a chemical substance, which does not require immobilization of the receptor or a special device, and a reagent to be used for this method. That is a method for assaying the receptor binding property of an assay target substance is provided, the method comprising the steps of (a) competitively reacting a known concentration of a ligand and the assay target substance with a known concentration of the receptor in a solution, (b) measuring, without physically removing the ligand bound with the receptor prior to the assay, the amount of a free ligand in the solution using one or more antibodies against the ligand, and (c) determining the receptor binding property of the assay target substance using the amount of the free ligand as an index.