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The oxidation of dimethyl-p-phenylenediamine through the cytochrome system is only very slightly increased by additions of cytochrome c (Borei & Renvall, 1949). It can be questioned whether the endogenous cytochrome c of the oxidase preparation, in mediating between the oxidase and the substrate, is sufficient to account for the whole oxidation, or if the reductant is directly attacked by the oxidase. In this investigation the question of the participation of cytochrome c has been studied for a number of reductants of the phenylenediamine type. The influence of additions of external cytochrome c has been recorded. The endogenous cytochrome c has been inactivated by low-temperature treatment. The toxicity of the reductants was investigated. MATERIAL AND METHODS Test 8y8tem. The oxidation of the reductant, in the presence of a cytochrome oxidase preparation and cytochrome c, was followed at 390 in differential manometers; shaking frequency 120 round trips/min.; total fluid volume 3 ml. The reductant (final concn. 0.03M) was tipped at the beginning of the experiments from a dangling cup into the main compartment. The velocity was obtained from the difference between the readings at 0 and 13 min. Phosphate

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