Five patients with insulin allergy were desensitized with porcine insulin according to the method of Urbach and Gotlieb. The diagnosis of insulin allergy was based on clinical manifestations, positive skin tests, and, moreover, on positive Prausnitz-Kustner (P.K.) reactions in 2 cases. Successful desensitization was obtained in 3 cases. Insulin antibodies were detected by the method of immunoautoradiography using131J-insulin. Before desensitization presence of IgG antibodies was found in 3 patients, and in 2 patients IgA antibodies were found also. The P.K. reactions were positive in patients whose sera contained IgA antibodies. During desensitization a transient appearance of macroglobulins binding131J-insulin was observed. After desensitization positive immediate skin responses to porcine and bovine insulin were found to persist. At the same time the delayed responses and the P.K. reaction to porcine insulin were reversed, although the same reactions to bovine insulin remained positive.