Ab8tractSheep uterine endometrial alkaline phosphatase in a purified state displayed optimal activity at pH 10·5. At pH 9· 0 activity increased with decreasing substrate concentration.The enzyme catalysed the hydrolysis of a variety of phosphomonoesters and exhibited pyrophosphatase properties when incubated with ADP and ATP. Metalcomplexing agents, such as EDTA, NaCN, o-phenanthroline, and L-cysteine inhibited the phosphatase while Mg2+ stimulated its activity. Recovery of activity was both rapid and complete when MgCb was added to the EDTA-inhibited enzyme suggesting that this alkaline phosphatase is a metalloenzyme with Mg 2+ as a functional component. No stimulating effect of EDTA was found at any concentration (lO-L lO-lM) used and substrate partially protected the enzyme against its inhibiting properties. Mg2+ in low concentrations (1 mM) increased the pyrophosphatase activity of the enzyme when incubated with ADP but decreased it at higher concentrations (2-8 mM). Mg2+ was more effective, however, in increasing the orthophosphatase activity of the enzyme and, in this case, did not inhibit activity at higher concentrations. High concentrations (0·1 and 1· OM) of KCI, NaCI, and NH4CI inhibited rather than stimulated activity, with the latter salt being the most effective. The enzyme was not stable to heating at 56°C and after 15 min at 65°C all activity was lost. The enzyme preparation was chromatographically and electrophoretically homogeneous and treatment with neuraminidase did not affect these properties. The enzyme exhibited phosphotransferase activity but was unable to catalyse the synthesis of glucose 6-phosphate by direct phosphorylation under the conditions employed.