Human red blood cells were separated according to density by centrifugation through mixtures of phthalate esters. The densest 20% of the erythrocyte population (old cells) had reduced volume and water content compared to the lightest 20% of the cells (young cells). Corpuscular hemoglobin content was unchanged. Young cells had 50% more potassium (K⁺) than old cells, but their total intracellular concentration was only slightly higher; old cells had a small increase in sodium (Na⁺) concentration. Active K⁺ transport of young cells was 37% higher than that of old cells. [³H] + Ouabain binding revealed that this difference was the result of more K⁺ pump sites on young cells, which bound 530 ouabain molecules per cell at 100% K⁺ pump inhibition, as compared to 400 for old cells; unseparated cells bound 450-500 molecules. The relative rates of ouabain binding were identical for the two cell types. Old cells exhibited a greater passive permeability to K⁺, haying a rate coefficient for ouabain-insensitive K⁺ influx 1.8 times that of young cells. There is evidence to suggest that in the face of reduced pump activity this augmented K⁺ “leak” might enhance the osmotic stability of the old cells and function to lengthen their life span.