This is the case of a 62-year-old man referred for the evaluation of insulin allergy. This patient had reacted to the subcutaneous injection of Novolin 70/30 (Squibb, Princeton, N.J.) and Humulin NPH (Eli Lilly, Indianapolis, Ind.). These reactions were characterized by the immediate onset of diffuse pruritic urticaria and angioedema with progression to hypotension as well as a local reaction. Past history also included anaphylactic shock after intravenous administration of protamine sulfate used for heparin reversal during arterial bypass surgery. Immediate hypersensitivty skin testing to protamine containing (NPH) insulin and protamine sulfate USP were strongly positive, while Lente insulin (Eli Lilly, Indianapolis, Ind.) and controls were negative. RAST tests revealed the titers > 24 ng/ml of protamine specific IgE with 98 percent inhibition and 1163 ng/ml of protamine specific IgG with 29 percent inhibition, while levels of insulin specific antibodies were negligible. Subsequently, the patient was treated with non-protamine containing insulin preparation, Lente insulin, without further incident. This study confirms the diagnosis of Type I hypersensitivity to protamine sulfate masquerading as insulin allergy.