Erythrocyte cation transport systems were evaluated on cell fractions from 17 subjects. Density centrifugation was used to separate washed red cells into fractions enriched with younger and older cells; the cell age differences in these fractions were verified by reticulocyte counts (means are 3.5% for younger cell fractions and 0.7% for older cell fractions). Red cell age has a pronounced effect on several cation transport activities. The older cell fractions had increases in lithium-potassium cotransport ( p < 0.001), the rate constant for the lithium-potassium cotransport ( p < 0.001) and cellular cation permeability. The older cells had decreases in the number of ouabain binding sites ( p < 0.001), the rate constant for sodium efflux via the sodium-potassium adenosine triphosphatase pumps ( p < 0.001) and the sodium-lithium countertransport ( p < 0.025). In subjects with markedly different cell ages, these effects should be considered when evaluating red cell cation transport activities.