The aim of the study was to compare the immunogenicity of semisynthetic human and porcine monocomponent (MC) insulin Novo during--5-year observation. Thirty-one diabetic patients, never previously treated with insulin were randomly allocated to treatment with one of the two coded insulin preparations in a double blind trial. 15 patients aged 20-58 years (mean age--36 years) were treated with human insulin and 16 diabetics aged 19-61 years (mean age--34 years) with MC pork insulin. The insulin was injected twice a day (Actrapid plus Monotard). Serum insulin-binding antibodies were determined according to Christiansen (radioimmunoelectrophoretic method). The development of insulin-binding antibodies during the first year of observation was more rapid in patients treated with MC pork insulin but thereafter it was similar in the two studied groups. After 5 years of treatment with human insulin serum insulin-binding antibodies were found in 14 patients. The level of antibodies was very low (< 0.130 mU/ml) in 5 subjects, low (0.171-0.401 mU/ml) in 6 patients and relatively high (0.885; 1.186; 5, 162 mU/ml) in 3 patients. In the group of diabetics treated with MC pork insulin after 5 years of observation serum insulin-binding antibodies were found in 12 patients. The level of antibodies was low (0.131-0.576 mU/ml) in 9 subjects and relatively high (1.034; 3.954; 5.639 mU/ml) in 3 patients. The results obtained after 2-5 years of the study did not differ significantly (Wilcoxon's test, p > 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)