On the basis of personal experience concerning 2020 consecutive determinations, the radioimmunoelectrophoretic method of Christiansen for 125I-Insulin-Binding to IgG (= IB, significant limits = mU/ml) has been proved as a reliable tool for the evaluation of insulin antibody titer in clinical diabetology. After a critical review of the recent literature about insulin antibodies both without and after exogenous immunization, the following results are presented and discussed. 1) - In 163 diabetic subjects, never previously treated with insulin, the mean value of IB was X = 0,008 mUml (sigma = 0,023 . Sx = 0,002). 2) - In 221 longterm insulin-treated diabetics the mean value of IB was X = 1,50 mU/ml (sigma = 2,15 . Sx = 0,145). 3) -In 46 insulin-dependent diabetics, serial determinations of IB allowed to follow the insulin antibody production during a 5 years treatment with monocomponent insulin )Lente MC). No or slight antibody formation was observed in newly diagnosed patients, never previously treated with insulin. High antibody starting levels showed tendency to a gradual reduction, after switching from conventional insulin treatment to the monocomponent one. These modifications in the IB values have been studied in correlation with the clinical course of conditions possibly referred to an immunologic pathogenesis, such as: brittle diabetes, high insulin requirement, insulin allergy, insulin lipoatrophy, diabetic microangiopathy. No significant variations in IB values were observed after viral infections.