Abstract When a purified bovine heart cytochrome c oxidase was polarographically assayed by a method involving initial contact of the enzyme with Tween 80, very high turnover numbers were observed at low enzyme concentrations, e.g. 5 to 10 mµm cytochrome a3. When substrate-reduced enzyme was treated with bathocuproine, the high turnover numbers at low enzyme concentrations were abolished but could be recovered by further treatment of the inhibited enzyme with Cui. This new form of inhibition of the enzyme, called the "bathocuproine effect," is ascribed to an interference by the reagent of enzyme dissociation to a unit aggregate of maximal activity. The effect is not due to copper depletion of cytochrome c oxidase.