METHODS

All chemicals were reagent grade made up in twice-distilled water. The saline was 145 mM aqueous sodium chloride solution made 0.3 mM with respect to sodium bicarbonate, pH 7.2 & 0.2. The phosphate-buffered saline (pH 7.2) consisted of 100 parts (v/v) of 145 111~ aqueous NaCl plus 5 parts (v/v) of phosphate buffer (8 parts of 67 mM aqueous Na2HPOd plus 2 parts of 67 mM aqueous KHzPOd). All solutions were in equilibrium with the atmosphere. Erythrocytes from fresh titrated human blood were washed (16) and suspended in either saline, phosphatebuffered saline, or saline containing 0.1 mM NazEDTA or 10% w/v low molecular weight dextran (M,-4 X lo4 Rheomacrodex, Pharmacia). Red blood cell suspensions of various volume fraction (H,) were prepared by mixing washed and centrifugally packed cells, with varying volumes of saline or saline containing 10% w/v Rheomacrodex. Acetaldehyde-fixed(hardened) red blood cells were prepared as described previously (16) and made up to various volume fractions (Hf) in the same way. Suspensions of red cells in plasma were prepared by mixing centrifugally packed cells, from which the buffy coat had been removed, with varying volumes of homologous ACD-plasma. Hematocrits were determined on both normal and hardened cell systems using Wintrobe tubes and centrifugation at 2,000 X