Redistribution of spin-labeled phospholipid analogues SL-PE and SL-PS in human red blood cells at neutral (7.4) and acidic (5.8) pH, respectively, of the suspension medium at 37°C. Erythrocytes were incubated for 5 minutes at 37°C with (in the case of SL-PS) or without DFP. 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). SL-PE: pH 5.8 (○), pH 7.4 (•). SL-PS: pH 5.8 (▿), pH 7.4 (▾). Curves were fitted by nonlinear regression to a monoexponential function (see the Materials and Methods). In the case of SL-PS redistribution at acidic pH, measured values were connected by straight lines (see Results).