Using isopycnic gradient centrifugation, blood samples from adults, from umblilical cord, from newborn infants and from infants of different age group up to the age of 8 mo, were separated into seven fractions according to cellular age. We measured the number of erythrocytes and reticulocytes, mean cellular volume, mean cellular hemoglobin concentration, and the concentration of hemoglobin in each of the seven fractions. The distribution of erythrocytes within the gradient during postnatal aging revealed a bimodal distribution between 2-4 mo. Correspondingly, during the age of 2-8 mo the oldest cells did not contain the highest hemoglobin F concentration as was observed in the younger age group. From these data we conclude that the postnatal disappearance of hemoglobin F is not a continuous process.