Summary The results in this experiment suggest that the increased serum alkaline phosphatase does not appear to be of liver or intestinal origin but may be of bone origin. Evidence in support of this finding has been obtained by the following: changes in alkaline phosphatase activity of various tissues during growth, measuring the change of enzyme activity and Ca45 concentration of various tissues after water dosing, enzyme properties of these tissues during ammonium sulfate fractionation and dialysis, and the chromato-graphic behavior of this enzyme from various tissues. It was also observed that the change of alkaline phosphatase activity in bone and blood is associated with the calcium metabolism.