The adaptation of cell physiological parameters of erythrocytes of the newborn to erythrocytes in later life was studied by cell separation in an isopycnic dextran density gradient. The cell characteristics during the first trimester of life were followed by repeated determinations. The mean cellular haemoglobin concentration of the lightest cells is remarkably less than that of the cells of median density. This is probably due to the ongoing synthesis of haemoglobin in reticulocytes, which are highly concentrated in the top fractions. It was shown that the young cells released from the bone marrow become progressively smaller during this period with a decrease of mean cellular volume (MCV) from 104 μ m3 at birth to 85 μ m3 at the end of the first trimester. It has been established that this process commences during the first week of life. No difference concerning the alterations of cell parameters, especially MCV, in premature and mature infants was found. It is therefore probable that these alterations are due to changes in the external milieu following birth.