Allergic reactions of the immediate type to insulin are known since its introduction of the treatment of diabetes mellitus. After the development of the chromatographically purified insulins they are to be found significantly reduced, though still particular cases are observed. The simultaneous occurrence of insulin allergy and insulin resistance seems to be not too rare. The immediate type allergy to insulin is mostly released by the presence of IgE-antibodies. Another but not at all rare allergic reaction is the Arthus phenomenon, released by IgG-antibodies. For diagnosis of these allergic states intradermal skin tests with various insulins are used. Also specific IgE-antibodies are to be measured radioimmunologically lately. There is a strong correlation between the outcome of the allergic reactions and the definite degree of the specific IgE-antibodies. Low levels of specific IgE are found in insulin treated diabetics without any visible allergic reactions to insulin. This fact proves that the hormone, though being a clean insulin of a very high degree, is the antigen itself. Not only immunologic mechanism, but also other exogenic influences cause allergic reactions of the immediate type to insulin.