The mechanism of activation of alkaline phosphatase (EC 3.1.3.1) from pig kidney by Mg(2+) ions was investigated with the aid of kinetic measurements. Mg(2+) ions are essential for enzyme activity. The following model (Scheme 1 of the text) for the reaction of enzyme, substrate and Mg(2+) ions was derived: [Formula: see text] The binding of the substrate to the enzyme is independent of the binding of the activator, and vice versa. Mg(2+) must therefore play a part in the substrate decomposition. It is not possible to determine whether the Mg(2+) ions are involved directly in the catalytic process, or whether they act as regulatory effectors. Because of the strong affinity existing between the alkaline phosphatase and Mg(2+), it is necessary to adjust the metal-ion concentration with the aid of a metal buffer. In the Appendix the necessary equations are derived for calculating the concentration of free metal ions in a system with several different metal ions. A FORTRAN IV program for solving these equations and for graphic presentation of the results has been deposited as Supplementary Publication SUP 50030 at the British Library (Lending Division) (formerly the National Lending Library for Science and Technology), Boston Spa, Yorks. LS 23 7 BQ, U.K., from whom copies may be obtained on the terms indicated in Biochem. J. (1973), 131, 5.