Immunization of rabbits using a peptide corresponding to the 12 amino acids of the C-terminal region of porcine insulin B-chain produced an anti-peptide antibody reactive with the native insulin, as well as with the peptide. It was also found that the adsorption characteristics of the anti-peptide antibody against insulin and peptides, including its reactivity against the insulin A-chain, were different from those of anti-native insulin antibodies. The reactivity against the insulin A-chain was raised by maturation of the anti-peptide antibody. These results show that peptide immunization may cause different immune-responses, and have some advantages for obtaining antibodies possessing desirable characteristics.