Abstract In the present paper, the important role played by cytochrome c in the cytochrome oxidase system is demonstrated by using highly purified praparatinos of cytochromes a , c 1 , and c 1 . Despite the fact that cytochrome a can be rapidly reduced by p -phenylene-diamine, the component is not oxidized by oxyge if cytochrome c is not present. The K m calculated for cytochrome c is similar to that found in particulate preparations. The addition of cytochrome c 1 has not effect on the cytochrome oxidase reaction. The reaction between cytochromes c 1 and a is very slow and is mediated by cytochrome c . On the basis of the above results, it is proposed that cytochrome c acts not only as an electron carrier between cytochromes c 1 and a but also as an essential constituent to form an active complex with cytochrome a in the cytochrome oxidase system. The reaction mechanism of teh cytochrome oxidase system is discussed on the basis of the above results.