Redistribution of the spin-labeled phospholipid analogue SL-PE in human red blood cells at various pH of the suspension medium at 4°C. After incorporation of SL-PE, the redistribution kinetics were monitored by the back-exchange of analogues on the outer leaflet to BSA (see the Materials and Methods). pH 5.8 (○), pH 7.4 (•), pH 8.5 (♦). Data represent the average ± standard error of estimate with n = 4 (pH 7.4 and pH 5.8), except for those measured at pH 8.5 (average of 2 independent experiments). Curves were fitted by nonlinear regression to a monoexponential function.