1. Immediately after coming into contact with glass, leucocytes are most adhesive at pH 8.0 or > 8.0.
2. Agglutination of leucocytes increases with increasing H ion concentration from pH 8.0 to 6.0.
3. In phagocytosis experiments where leucocytes creep about on the slide picking up articles the optimum pH is 7.0. Here ameboid movement is probably the limiting factor.
4. The optimum for phagocytosis of quartz from suspension is on the acid side of neutrality at or near pH 6.7.
5. Phagocytosis of quartz increases with the acidity, while adhesiveness of leucocytes to glass increases with the alkalinity.