PURPOSE: To obtain an idioantibody of cholecystokinin-pancreozymin (CCK- 8-P) by a method wherein a conjugated antigen of the end peptide of CCK-8-P and a carrier is given to a mammal which is made unresponsive immunologically to gastrin.

CONSTITUTION: A combined substance of β-alanyltetragastrin and D-glutamate- D-lysine copolymer, which is expressed by the formulaI, is given to a mammal to make it unresponsive immunologically to gastrin and compounds similar to the gastrin. A conjugated antigen of the C-end peptide of cholesystokinin-pancreozymin (alimentary canal hormone) and a carrier (natural or synthetic protein or the like), expressed by the formula II, is given to the mammal and an antibody thus produced is collected. This antibody has a high peculiarity to CCK-8-P having the same arrangement as five amino acids at the C end of the gastrin, and enables the accurate quantification of CCK-8-P in a humor wherein the gastrin and the compounds similar thereto exist.