Erythrocyte ghosts of male persons of two age groups, younger than thirty and older than seventy years, were analyzed to investigate the imfluence of age on lipid composition, the physical state of the lipid phase, Na+K+-ATPase activity and sialic acid content. The phospholipid content in red cell membranes of old donors is significantly lower than in young ones. Cholesterol and fatty acid compositions shows no difference between the two donor groups. The membrane fluidity in liposomes prepared from total lipids of old donor decreased. No significant difference in lipid composition and membrane fluidity reflected by the spin labels was observed between blood group A and O. The activity of Na+K+-ATPase of the young donors with blood group A is significantly higher than those of old donors. The results also demonstrate a decrease of sialic acid content of red cells of old donors.