1. It has been proved that the technique of Takamatsu and Gömorri indicates the cytological sites of alkaline phosphatase activity.
2. The critical time limits for the various operations in the technique have been determined.
3. Several alternative techniques are given for demonstrating phosphatase activity on the cytological level.
4. A selection of fixatives is given which do not produce severe destruction of alkaline phosphatase activity.
5. It has been shown that alkaline phosphatase in fixed perparations is found in sites which are those in which it occurs under physiological conditions.
6. It is suggested that the function of the phosphatase found in the brush borders of the proximal tubules of the kidney is to hydrolyse phosphate esters in the glomerular filtrate, and thereby permit the reabsorption of the component molecules by tubular processes.