1. An accurate quantitative method of measuring the penetration of dye into the living cell is described.
2. Cresyl blue is unable to penetrate rapidly unless the pH outside the cell is decidedly greater than that inside. The rate of penetration increases with increasing pH.
3. Around pH 9 penetration of the dye is rapid while the reverse is true of exosmosis. At low pH values (5.9) exosmosis is rapid and penetration is very slow.