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Scant attention has been given to osmotic changes occurring in erythrocytes stored at lon-teiiiperatures although most workers insist upon the use of isotonic solutions in the preservative fluid. Rous and Turner' found no relationship between the resistance of red cells to hypotonic saline solutions and to niechanical shaking. Several investigators"-" have reported increased fragility of the erythrocytes during storage. Maizels and Whittaker' noted that although blood stored in hypertonic saline solutions showed little hemolysis the erythrocytes were ruptured v-heii placed in normal plasma because their contents had beconie equilibrated to the hypertonic plasma. The permeability of red cells to dextrose suggested the possibility of a similar reaction when that sugar is used in the preservative. The present study is concerned with certain phases of this problem. Table I presents the data from 3 representative experiments. Changes in cell volume were calculated from hematocrit readings. The fragility of the erythrocytes was estimated by adding 0.1 cc of whole blood mixture to 2.0 cc of each of a series of sodium chloride solutions with concentrations ranging from 0.24% to 1.20% in increments of 0.04%. After 1 hour at room temperature, the highest concentrations showing initial and complete hemolysis were noted-values are expressed in that order. For testing washed cells the plasma was removed after centrifugation and the cells washed twice with 0.95 % sodium chloride solution before being added to the series of saline solutions.In citrate-blood there was a progressive increase in cell volume during storage which was approximately paralleled by increase in fragility of both washed and unwashed cells. I n dextrose-citrate-