The aim of the present study was to compare the immunogenicity of monocomponent human insulin with that of monocomponent porcine insulin in newly diagnosed Type 1 (insulin-dependent) diabetic children. One hundred and thirty-five patients at diagnosis of diabetes (age 1-18 years, mean age 9.3 years) were randomly allocated to treatment with either Monotard MC + Actrapid MC or Monotard HM + Actrapid HM in a double-blind trial conducted in Sweden, Finland, Norway and Denmark. The human and porcine insulin groups were identical at diagnosis with respect to age, sex and measures of metabolic control. At all times the mean insulin antibody levels and the percentage of antibody-positive patients were lower in the human group compared with the porcine group. At 3 and 12 months, the insulin antibody values were significantly lower in the human group than in the porcine group (p less than 0.05, Wilcoxon's rank sum test). At 12 months, antibody values in the human group ranged from 0 to 1.2 U/l (mean 0.14 U/l) and in the porcine insulin group from 0-5.2 U/l (mean 0.63 U/l). It is therefore concluded that human monocomponent insulin has a lower immunogenicity than porcine insulin of the same purity in newly diagnosed diabetic children during the first year of insulin treatment.