Glycosylation of human serum albumin (HSA) with ascending glucose concentrations was studied. The incubation medium contained 5% HSA and glucose in concentrations 11, 22, 44, and 88 mmol/liter. The degree of glycosylation was assessed by the end product, fructosamine. A linear correlation between glucose concentration and fructosamine was detected. Addition to incubation medium of human native plasma of a normal subject or diabetic with type I disease (50% of the initial volume) reliably inhibited glycosylation. Thermostable low-molecular substances were identified, which cause a delay of glycosylation: creatinine, uric acid, and arginine. Addition of these compounds in physiological concentrations reliably reduced glycosylation of HSA. Hence, we found the components of blood plasma defense system which prevent the formation of fructosamine in hyperglycemia.