It was previously shown that the addition of cytochrome
c
to a heart-muscle preparation greatly increases its power of catalysing the oxidation of such substances as
p
-phenylene diamine, hydroquinone, cysteine and ascorbic acid. It was demonstrated that the oxidations of these substances, which can be used for the detection and estimation of intracellular oxidase, are catalysed not directly by the oxidase but through the co-operation of cytochrome. The only direct function of the oxidase, so far ascertained, is the oxidation of reduced cytochrome, and the enzyme can therefore be considered as cytochrome oxidase (Keilin and Hartree 1938
a
). Several properties which have been previously ascribed to it do not complete cytochrome-oxidase system. The object of this paper is the study of the mechanism of oxidation and reduction of cytochrome in order to determine the properties and the nature of cytochrome oxidase.
