The activities of alkaline phosphatases of kidney and urine appear similar and identical in action. Both are magnesium-dependent and inhibited by higher concentrations of 2-glycerophosphate. A band of similar mobility is obtained on electrophoresis of kidney and urine enzymes. In vitro magnesium competitively inhibits their activities when the ratio of Mg^+2 ions/ 2-glycerophosphate is above a definite level. Their activities are not affected by exogenous zinc and follow the Michaelis-Menten equation only when attention is given to the ratio of Mg^+2ions/substrate. Nitrofuranfuradroxyl is a mixed type inhibitor showing a second-order rate of reaction with kidney and urine phosphatases.