1. The effect of variations in extracellular osmolality from 249 to 345 mosmol/kg upon the sodium, potassium and water content of human leucocytes has been studied.
2. Similar studies were performed using human erythrocytes.
3. Changes in the leucocyte water content were not explicable in terms of passive movement of water across an ‘ideal’ semi-permeable membrane.
4. Hypo-osmolal swelling was associated with a rise in intracellular sodium content and hyperosmolal shrinkage was associated with a fall in intracellular sodium content.
5. There were no significant changes in sodium and potassium content of the erythrocyte with altered external osmolality.