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Our interest in erythrocyte shape changes originated several years ago when we began to look at some of the functional and structural alterations in erythrocyte membranes that attend the aging cell.' The erythrocyte has a finite lifetime which varies according to species, and the age of the cell at death is somewhat related to the size of the animal; generally, the larger the animal the longer its red cell lifetime.* Reptiles have red cell lifetimes on the order of several hundred days while birds fall into the mammalian category. It has been proposed that the red cell lifetime is a function of the oxygen-carrying capacity of the erythr~cyte,~ a view which suggests that the lifetime of the cell is regulated either by oxygen or by a product of oxygen-carrying capacity. Possibly the production of peroxides, free radicals and their covalently modified products function as a molecular biochemical clock which in part regulates the finite lifetime of the cell.A number of significant modifications ensue as the cell becomes older; these include the loss of cell def~rmability,~ increased fragility of the erythrocyte and increased sensitivity to hemolysi~,~ and increased cell density.6 Investigators have suggested numerous mechanisms as possible culprits in the death of the erythro~yte.~-"' Recent investigations in our laboratory, '.''-I3 carried out in rat erythrocytes, have directed attention to changes in the membrane proper and its associated cytoskeletal assembly. We have shown that a number of functions and properties of erythrocyte membranes change as a function of cell age. Included among these are changes in catalytic properties, protein composition, phospholipid composition, and the interaction of the membrane and cytoskeletal matrix. The age-related increase in cell density has been exploited'.l4 to separate cells on the basis of their age by centrifugation techniques. The age of separated cells can be ascertained with methods that utilize 59Fe, 3H-DFP, or 51Cr. The cell separation techniques are not absolute, but result in relative enrichment in the age of a population of erythrocytes which lend themselves to comparison by biochemical and biological analyses.A number of biochemical reactions have been described in erythrocyte membranes, which while not completely understood, suggest that these reactions are involved in the regulation of the moment to moment activities of the cell, for example, its shape's.'6 and in the determination of its life span. Among these are