Separation of erythrocytes into fractions of different age was carried out by the method of discontinuous gradient. Five fraction of various specific density were prepared and in each fraction age dependent enzyme activities PK and G-6-PD were assayed. The erythrocytes from cord blood and from newborn 4-5 days old were studied. The results demonstrate the correlation between density and erythrocyte age in both cord blood and blood from newborns. However, 4-5 days old demonstrate a shift of various populations toward fractions of higher density.