Using a quantitative absorption technique, it was shown that the concentration of H-2 antigens on the surface of spleen and liver cells of newborn A and C3H mice increased during the first days after birth; adult level was reached after 5 days. Similar cells from strain C57BL reached adult level after 2 days. A study on the presence of individual H-2 antigens indicated that they were all present to a detectable degree from the day of birth. The absorbing ability of the newborn spleen and liver cells showed an increase that paralleled their cytotoxic sensitivity. Since all newborn spleen cells contained H-2 antigens and since all individual H-2 antigens could be demonstrated on the cells, the absorption capacity reflects the concentration of isoantigens. It appears, therefore, that the increase in the cytotoxic sensitivity of newborn spleen cells is closely connected to the increase of the concentration of isoantigenic surface receptors. This conclusion substantiates earlier findings on tumor cells.