The antigenic properties of insulin and the effect of additional factors on them, HL-A-phenotype including were studied under different conditions in 3 groups of diabetic patients (I--71 patients treated with insulin, II--48 diabetic patients--never treated with insulin and III--51 diabetics, treated at least 6 months with conventional insulin and later treated with monocomponent insulin "Actrapid"). In some other experiments, making use of 7 groups guinea pigs, the antigenicity of different trade insulin preparations was tested. The antibody titre was assessed by the insulin-binding serum capacity, incubated with 131I-insulin. The authors established a significant dependence of antibody titre on insulin during insulin treatment and insignificant--with the age of the patients, duration of insulin treatment and HL-A phenotype. The last correlation with HLA-phenotype B 15 was highly significant. A reduction of antibody titre was established after changing from conventional to highly-purified monocomponent insulin preparation and anew elevation of titre with the resumed treatment with non-purified insulin forms according to special ways. All insulin preparations, that the separate groups of guinea pigs were injected, led to significant increase of antibody titre in the experimental animals. The better purified of them, however, as "insulin Comb" and Bulgarian "neutral porcine insulin"--Pharmachim, are less antigenic. A treatment with highly purified preparations is recommended for the diabetic patients with HLA-phenotype B 15.