Since alkaline phosphatase from Escherichia coli was first reported to contain 2.1 g-atoms of zinc and 0.8 g-aton of magnesium per molecular weight 80,000 (Plocke, D.J., Levinthal, D., and Vallee, B. L. (1962), Biochemistry 1, 373-378), the procedures for isolation and purification of the enzyme, as well as values for the protein molecular weight, specific absorptivity, and maximal activity, have changed repeatedly. Such variations have resulted in uncertainties concerning the molar metal content of this phosphatase. The present paper reviews the initial and recent results of metal analyses of alkaline phosphatase preparations in this laboratory and compares them with those obtained elsewhere, while simultaneously identifying some of the factors which have affected reports on the metal content of this enzyme. A purification procedure is described eliminating the features of all methods known to alter the metal content of phosphatase. In addition, the three isozymic forms, as well as preparations from four E. coli strains commonly employed for phosphatase isolation, were analyzed and compared.