Redistribution of the spin-labeled phospholipid analogue SL-PE in human red blood cells resuspended in buffer systems of different chemical nature. After incorporation of SL-PE, the redistribution kinetics were monitored (4°C) by the back-exchange of analogues on the outer leaflet to BSA (see the Materials and Methods). pH 5.8: MBS (○), PBS (□), MBS/HBS (▵). pH 7.4: HBS (•), PBS (▪), MBS/HBS (▴). Curves were fitted by nonlinear regression to a monoexponential function.