Peroxidase activity in Walker carcinoma 256 of rats has been demonstrated. For full activity, it is necessary to inhibit the catalase present with 2,4-dichlorophenol. A method for the cytochemical fractionation of this tumor tissue has been presented, and it was demonstrated that peroxidase is located principally in the mitochondria. Some properties of the enzyme have been presented, and the possible role of peroxidase has been discussed.

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Supported by a grant from the Jane Coffin Childs Memorial Fund for Medical Research.

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