1. Michaelis constants, maximum velocity and pH-dependence of the reaction catalysed by homogeneous AMP-deaminase preparations from hen, frog and pikeperch skeletal muscle were compared, as well as the influence of monovalent cations, ATP and inorganic phosphate. 2. ATP was found to activate the enzymes in the absence of K+ and at optimum (150 mM) KCl concentration. 3. Absolute dependence on potassium ions and considerable dependence of Km and Vmax on the kind of monovalent cation present in the medium were found for pikeperch enzyme.