The immunogenicity of conventional therapeutical insulin is discussed according to the concepts of Schlichtkrull: the formation of insulin antibodies is not attributable to the pure Sanger's insulin molecule, but to related protein impurities, present in all crystallized pig and ox insulin preparations. The terms of monocomponent insulin, highly purified insulin, and single peak insulin in defined and personal clinical results obtained with Novo Monocomponent Lente Insulin over a period of 3 years are presented. The Hein Christiansen's radioimmunoelectrophoretic method fo estimation of 125I-insulin IgG binding was used to determine insulin antibody levels. It was found that: 1) Newly detected insulin-dependent diabetics, never previously treated with insulin, do not produce insulin antibodies at a significant level; 2) Long-term insulin treated diabetics, transferred to monocomponent treatment, tend to reduce their antibody levels, if initially high, altough with transient recurrent peaks; 3) Stimulation of the immunocompetent system by intercurrent infection does not generally modify the immunological situation. Apart from immunological changes, satisfactory clinical results were observed in cases of high insulin requirement, insulin allergy, insulin lipoatrophy. Present practical indications for monocomponent insulin therapy (Actrapid-Lenta) are proposed.