Using an enzyme-linked immunosorbent assay method, we have demonstrated immunoglobulin E (IgE) and immunoglobulin G (IgG) against human (recombinant DNA) insulin in serum samples of a patient whose insulin therapy was only with human insulin. Within 12 days of initiation of human insulin for gestational diabetes, large local reactions developed in the patient in association with high levels of IgE to human insulin. As the local reactions subsided, the levels of IgE to human insulin decreased. The patient's insulin requirement was unusually high for pregnancy and tended to decrease in parallel with falling levels of IgG to human insulin, suggesting an element of immunologic insulin resistance. In addition to IgE and IgG against human insulin, we were able to demonstrate the presence of IgE and IgG against porcine and bovine insulin in the serum samples of this patient. The patient had similar immediate cutaneous reactivity to each insulin. In inhibition studies, similar amounts of porcine insulin, bovine insulin, or human insulin were found to produce 50% inhibition of human insulin binding to IgE or IgG against human insulin. Excess porcine insulin or bovine insulin resulted in total inhibition of human insulin binding to IgE or IgG. Thus even in a patient whose initial form of insulin therapy was human insulin, there was development of insulin allergy and of IgG to human insulin that may have contributed to insulin resistance. Moreover, the antigenic determinants recognized by antibody induced by human insulin may also be present on bovine insulin and porcine insulin as demonstrated in this case.