The invention provides a novel 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. Specifically, the invention provides a stabilizing agent for a specific-binding-pair-forming substance, the agent containing 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 containing 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 containing a stabilizing agent or a blocking agent for preventing non-specific adsorption.