Sodium pump function has been studied in human erythrocytes depleted of membrane cholesterol by incubation with phosphatidylcholine liposomes. The cells were sodium loaded by incubation in alkaline sodium phosphate and sodium pump activity was assessed by measurements of ouabain-sensitive 86Rb uptake at 37 degrees C. Cholesterol depletion had a biphasic effect; depletion by 5-25% increased sodium pump activity by a mean of 16.1% (S.D. 3.2%), whereas depletion by 35-50% decreased sodium pump activity by a mean of 14.8% (S.D. 3.8%). Cholesterol depletion had no reproducible effect on the ouabain-insensitive uptake of Rb. These results support the hypothesis that there may be an optimum membrane cholesterol content for sodium pump function.