The activation of mitochondrial ATPase by 2,4-dmitrophenol in a sucrose medium was considerably increased in the presence of chloride ions. The effect of this anion was independent of the form in which it was added (monoethanolamine, diethanolamine, Tris, choline, sodium, or lithium chloride), with the exception that the potassium and rubidium salts were slightly more active.The effect of chloride ions was compared with that of other anions. It was found that the activity of each anion could be related to its strength as a base.