Plasma and erythrocyte lipid levels and susceptibility of erythrocytes to lipid peroxidation were determined in rabbits fed diet containing 2% (w/w) cholesterol, for 3 months. Hypercholesterolemic rabbits had high plasma and erythrocyte lipid peroxide levels as compared to control rabbits. After high-cholesterol diet, the rabbits in the experimental group were divided into two groups. The first group was fed a normal diet for 21 days and the second group was given normal diet plus thymosin F5 injections every other day for the same period. At the end of this period, plasma and erythrocyte lipid peroxide levels were significantly decreased in the group injected with thymosin F5.