Redistribution of spin-labeled phospholipid analogues SL-PE and SL-PS in ATP-depleted human red blood cells at neutral and acidic pH, 7.4 and 5.8, respectively, of the suspension medium at 4°C. ATP depletion was performed by incubation of red blood cells in the presence of iodoacetamide. After incorporation of spin-labeled phospholipids, the redistribution kinetics were monitored by the back-exchange of analogues on the outer leaflet to BSA. SL-PE: pH 5.8 (○), pH 7.4 (•). SL-PS: pH 5.8 (▿), pH 7.4 (▾). For details, see the Materials and Methods. Curves were fitted by nonlinear regression to a monoexponential function.