Some indices of free radical processes and antioxidant blood protection systems were investigated in a group of persons exposed to ionizing radiation at the Chernobyl accident. Lipid peroxidation was shown to undergo changes against a background of decreased antiperoxide protection of erythrocytes 22-24 mos. after exposure in patients after acute radiation sickness, I degree. As compared to donors, the examinees demonstrated a tendency to the accumulation of primary products of lipid peroxidation--hydroperoxides (by 32.2%) and a significant decrease in erythrocyte peroxide stability.