A study was made of peroxidation of lipids of erythrocytic biomembranes in 42 patients with chronic glomerulonephritis: 23 patients with the urinary syndrome and 19 with the nephrotic syndrome. Activation of free radical lipid oxidation was shown in both groups of patients; it was especially marked in the group with the nephrotic syndrome. Three degrees of the activity of a renal process were defined on the basis of difference in the concentration of secondary peroxidation products in concrete patients. Correlation between lipid peroxidation indices and expression of proteinuria and leukocyturia was established.