ABSTRACT The investigation shows that in a group of fifty-one patients with diabetes mellitus treated with pure porcine insulin preparation, administered as crystal suspension of protamin-insulin, 76 per cent formed antibodies reacting with insulin. None of the patients in the observation period developed insulin resistance. No sex difference was demonstrated in the antibody formation, similarly, it was not possible to demonstrate a stimulation or accentuation in the antibody formation in connection with catarrhal infections. It was demonstrated that younger patients formed more antibodies than the older patients. Furthermore, a correlation was demonstrated between the antibody titre and the insulin dosage/kg weight. With the method used it was possible to demonstrate antibodies after 1–3 months of insulin treatment. The antibody titre attained a maximum level after insulin treatment for 4–9 months, after which it remained unchanged, probably as a consequence of afferent enhancement. It was found that the magnitude of the antibody titre was probably secondary to the size of the insulin dosage, and that because of the close correlation between regulation and insulin dosage, the influence of inadequate regulation could not be excluded. On the other hand, the magnitude of the antibody titre seemed to be without any importance for the regulation of diabetes, and in non-resistant diabetics to be without importance in determining the size of the insulin dosage.