Thirty-one previously untreated diabetic individuals received only human insulin (recombinant DNA) for 1 yr with no adverse reactions. The development of serum IgE antibodies to human, porcine, and bovine insulins was assessed by a sepharose radioallergoabsorbent test (RAST). Immunoglobulin (total Ig antibody) binding was assessed by a nonabsorbed species-specific radioimmunoassay. During therapy 2 patients developed IgE antibodies to human insulin as well as increased total Ig binding. The IgE antibodies to human insulin cross-reacted with porcine and bovine insulins, were transient, and were not accompanied by insulin allergy. Ig binding to insulin developed and persisted in 11 of the human insulin-treated diabetics. In comparison, 62 previously untreated diabetic persons received only purified porcine insulin (PPI, less than 5 ppm proinsulin, N = 40) or a mixed bovine-porcine insulin (proinsulin less than 50 ppm, N = 21). Increased Ig antibody developed in 16 of 21 patients receiving mixed bovine-porcine insulin and 25 of 41 PPI-treated patients (P less than 0.05). Seven of 41 PPI-treated patients and 4 of 21 mixed bovine-porcine-treated patients developed anti-insulin IgE antibodies, which were transient in 4 and persisted in 6 diabetic patients. IgE antibody levels did not correlate with total Ig antibody. These data suggest that IgE and total Ig antibodies develop less often after human insulin treatment. Also, the immunoregulation mechanisms responsible for anti-insulin IgE antibody synthesis differ from those regulating other Ig that bind to insulins. Since none of the patients in this study have developed clinical manifestations of insulin allergy or resistance, the clinical relevance of the antibody data must remain speculative.