Fasting concentration of the C peptide in serum was estimated in 150 patients with type 2 diabetes treated with insulin because of the late, true ineffectiveness of the sulphonylurea derivatives. In 36 patients selected out of the total group at random the secretion of that peptide was measured after i.v. injection of 1 mg of glucagon. Only 9 patients showed trace amounts of that peptide at morning fast (Group A--0.17 +/- 0.08 nmol/l), in 69 the secretion was normal (Sub-Group B1--0.80 +/- 0.25 nmol/l), in 48 moderately elevated (Sub-Group B2--1.67 +/- 0.10 nmol/l) and in 24 markedly elevated (Sub-Group B3--4.54 +/- 2.57 nmol/l). The increments of the peptide C concentration after glucagon stimulation were parallel to its fasting concentration, which indicated a proper reactivity of the pancreatic beta-cells in patients with normal or increased basal secretion. The patients with only trace secretion of the peptide C differed from the other by their small, normal body mass and by a longer duration of insulin treatment. Very similar insulin needs must be stressed in the patients of the Groups A and B as well as within the Sub-Groups B. In patients with hyperactivity of the beta-cells (Sub-Group B2 and B3) no differences were found, as compared with the other patients, in the prevalence of chronic diabetes complications of the micro- or macroangiopathy type, also prevalence of hypertension was equal. The results presented show that in the most patients with type 2 diabetes, with the late, true ineffectiveness of the sulphonylurea derivatives the secretory function of the pancreatic islets beta-cells remains normal or is even increased.