A simple quantitative procedure is described for estimation of nonenzymatically glycosylated hemoglobin (HbAIc) in blood, which involved acid hydrolysis of hemoglobin in trichloracetic acid with heating. The liberated fructose was measured by development of colour complex of 5-hydroxymethyl furfurol (produced from fructose in acid solution) and thiobarbituric acid. Estimation of HbAIc was carried out in 30 healthy persons and in 50 patients with diabetes of the I type using the procedure developed. Concentration of HbAIc constituted 3.0 +/- 0.15% of total Hb in healthy persons, while its content was increased up to 7.0 +/- 0.5% in patients with diabetes of middle severity and up to 20% and higher in patients with severe forms of the disease. The procedure developed is recommended for use in laboratories of wide profile for diagnosis of diabetes.