The present invention provides an agent for improving the blood stability of an endogenous ligand, which comprises an antibody that has affinity to the mammalian endogenous ligand but substantially does not neutralize the same, and the above-described agent for the prophylaxis and/or treatment for a disease for which an increase in the blood concentration of the endogenous ligand and/or an prolonged blood half-life is prophylactically or therapeutically effective. Provided that the above-described agent is administered alone to a mammal without co-administering the same or substantially the same compound as the endogenous ligand, the blood stability of the endogenous ligand increases and the receptor activity-regulatory action thereof is enhanced.