The immunologic properties of homologous and heterologous insulins have been investigated. Pigs, dogs, cows, sheep, goats, rabbits, guinea pigs, and rats were immunized with different hormone preparations alone or in combination with complete Freund adjuvant. The results obtained provide convincing evidence that in pigs homologous insulin cannot produce specific antibodies, whereas heterologous insulin can. Because the insulins of dogs and pigs have identical amino acid sequences, no antigenicity of porcine insulin in dogs could be observed either. In cattle, sheep, and goats, not only heterologous but also homologous insulins stimulated antibody production. Sheep and goats proved to be excellent reactor animals. Most of the small laboratory animals developed specific antibodies against insulin after hyperimmunization. In rabbits, not only the groups injected with nonchromatographed bovine insulin but also those hyperimmunized with single-component bovine insulin responded with a high serum level of specific antibodies. The data suggest that highly purified insulin preparations have not less antigenic activity than nonchromatographed insulin. Immunologically, des-Phe-B1-insulin acted exactly like the original hormone. Histologic examination of the pancreases of 45 pigs, 22 of which had high antibody titers, did not reveal insulitis. The results of the present paper point out that the production of specific antibodies is essentially a question of species specificity.