A stabilizing agent for enhancing stability of a specific-binding-pair-forming substance during storage and a novel blocking agent for preventing non-specific adsorption for use in an assay employing a specific-binding-pair-forming substance. The agent may contain a plant-derived polypeptide as an active ingredient a blocking agent for preventing non-specific adsorption in an assay employing a specific-binding-pair-forming substance a composition for use in an assay employing a specific-binding-pair-forming substance, the composition may contain the specific-binding-pair-forming substance and the stabilizing agent or the blocking agent and a kit for use in an assay employing a specific-binding-pair-forming substance, the kit may contain a stabilizing agent or a blocking agent for preventing non-specific adsorption.
