The tonicity-volume relations for systems containing human red cells and the chlorides of monovalent cations.

1. Differences in the fragility of human red cells are observed in equimolar solutions of the chlorides of the monovalent cations. The cells are most fragile in LiCl and least fragile in NaCl, the salts falling in the order Li > K >== Rb > Cs > Na. 2. The difference between the tonicity-volume relations in systems containing LiCl and systems containing NaCl lies in the molarity of the solution of LiCl which is isotonic (isoplethechontic) with plasma being considerably greater (0.189 M) than the molarity of the solution of NaCl which is isotonic (isoplethechontic) with plasma (0.160 M). The difference cannot be stated meantime in any simpler terms than these; if the activity coefficients are taken into account, it becomes even greater. The tonicity-volume relations for the two systems are otherwise almost identical; the value of R for the two systems is almost the same, the critical volumes at which the cells of least resistance hemolyze are almost identical, and the critical volumes at which the cells of average resistance hemolyze are almost identical. 3. The LiCl effect on volume occurs as soon after the addition of the cells to 0.172 M LiCl as the hematocrit method allows one to measure it. It is reversible by washing with 0.172 M NaCl.