A comparison is made of the staining results obtained with Nitro-BT and MTT-Co++ as acceptors when DPNH is the substrate in frozen sections of cold formol-calcium-fixed rat kidney (normal and following ligation of the blood vessels) and human liver containing lipofuscin granules. The kidney results are evaluated in terms of mitochondrial morphology seen after classical mitochondrial stains and in electron micrographs. It is concluded that, except for formazan deposition on lipid-aqueous interfaces, Nitro-BT staining indicates the intracellular localization of oxidative enzymes, at least at the level of light microscopy. In contrast, the use of MTT-Co++ is not reliable for such intracellular localizations. The deposition of the formazan of MTT-Co++ is determined in large part by physicochemical factors other than enzyme localization. Despite marked abnormalities of the mitochondria in cells of the ligated kidney, MTT-Co++ formazan is generally deposited in the same dotlike fashion as in cells of normal kidney.

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