The changes in hematological parameters and physicochemical composition of red cells which accompany their aging in vivo and the results of recent experiments in which the peripheral zone of density fractionated erythrocytes were labeled with positively charged colloids or treated with relatively high molecular weight polycations suggest a reordering or restructuring of membrane components of red cells, rather than a change in their mean surface charge density. Age-related alterations in the two-dimensional arrangement of integral membrane glycoproteins may arise as a result of membrane loss. These types of perturbations may also be responsible for other altered properties of aged cells as seen, for example, in their behavior in two-phase aqueous polymer systems [30], their agglutinability in the presence of certain antisera [4], their permeability to ions [17], and mechanical properties of the membrane [32].