The kinetic, electrophoretic and immunological properties of hexokinase from human erythrocytes have been studied in relation to cell age. No differences in kinetic behaviour between hexokinase partly purified from reticulocytes, 10% youngest cells, normal red cell population or from 10% oldest cells were observed. The stability of the enzyme preparations showed little differences; hexokinase from the 10% youngest cells was the most labile enzyme, followed respectively by the enzyme from reticulocytes, normal red cell population and the 10% oldest cells. The electrophoretic pattern of erythrocyte hexokinase changed during senescence. The hexokinase activity located in the second band from the anode is shifted to the third with increasing cell age. The molecular specific acitivity of the enzyme from the 10% youngest cells, the normal red cell population and the 10% oldest cells remains the same, while the molecular specific activity of hexokinase from reticulocytes was much lower.