A change occurs in chicken erythrocytes during suspension in salt solutions or solutions of nonelectrolytes in Locke-Ringer solution with an osmotic pressure three to four times that of the original cells. If the cells shrink and then swell in less than 10 min this change does not occur. The changed cells are more fragile than normal cells. They have less potassium but more sodium. Evidently when these erythrocytes shrink to a certain point there is some irreversible change. The mechanism for maintaining the cation imbalance between the cell and its environment can break down, and the cells can lose potassium and gain sodium. The cells are highly fragile, and consequently they hemolyze under various circumstances which are not hemolytic to normal cells. Submitted on November 14, 1960