The role of pH on tissue alkaline phosphatases have been investigated in different human tissues. Different isoenzymes have been observed in different organs. Bone and liver were similar in having the isoenzymes optimally active at 9.4 and pH 9.9. The kidney has a single alkaline phosphatase optimally active at pH 9.2, while the adrenal gland has two isoenzymes of optimum pH 9.2 and 10.1. Intestinal alkaline phosphatase was optimally active at pH 9.7.