Redistribution of the spin-labeled phospholipid analogue SL-PE in human red blood cells at various pH of the suspension medium at 4°C. In part of the experiments, the pH of suspension medium was changed during the translocation measurements. After incorporation of spin-labeled phospholipids, 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 7.4 → pH 5.8 (▿) at 120 minutes, pH 5.8 → pH 7.4 (▾) at 120 minutes, pH 7.4 → pH 5.8 (▿) at 480 minutes, and pH 5.8 → pH 7.4 (▾) at 480 minutes. Curves were fitted by nonlinear regression to a monoexponential function.