AbstractTheoretical osmotic fragility curves were calculated and drawn by computer using the van't Hoff equation and the isotonic areas and volumes of 1000 individual erythrocytes. We studied the influence on the calculated curves of theoretically altering the fraction of the volume which was osmotically active from 50 to 70, and of altering the permissible stretch before hemolysis from zero to 10. With the two assumptions–that the membrane does not stretch before hemolysis, and that the osmotically active fraction of the cell volume is 0.58–it was possible to duplicate the general shape of the standard fragility curve; the exact NaCl concentration, however, at which there was 50 hemolysis was approximately 0.1 gm/100 ml higher than foundin vitro. The calculated osmotic fragility curves can be made quantitatively similar toin vitroones if the following statements are true: the osmotically active volume is 58, the permissible stretch of the membrane without lysis is 6, the cell membrane resists a slight osmotic pressure gradient of approximately 0.1 atmospheres, and hemolysis is an all or nothing phenomenon. This set of values for the relevant factors is sufficient but not unique in causing the superposition of the calculated and experimental curves. The frequency distribution of the cells according to the hemolytic salt concentrations (the sodium chloride concentration at which an individual cell just hemolyzes) was skewed positively and was leptokurtic for each of the seven normal subjects stud
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