Concentrations of BaCl(2), MgCl(2), SrCl(2), and CaCl(2) can be found in which the volume of washed human red cells remains almost unchanged for short periods of time; in more concentrated solutions the cells shrink, and in less concentrated ones they swell. Between tonicities of about 1.5 and 0.75, the van't Hoff-Mariotte law applies roughly, but at lower tonicities the red cell volume is anomalously great, sometimes in the absence of hemolysis. 2. If the cells are allowed to stand at 4 degrees C. in the media of different tonicities, the volume changes are not maintained. The volumes decrease in a complex way, and the decreases are accompanied by a loss of K from the cells and an entry of the external cation into them. 3. With two exceptions, these ion exchanges are not accompanied by any important changes in the osmotic, mechanical, or heat fragility of the red cells. The exceptions are a marked effect of BaCl(2) on heat fragmentation, and of CaCl(2) on osmotic and mechanical fragilities.
