Electron micrographs of a purified succinate and DPNH oxidase system prepared from heart muscle reveal that it has a vesicular appearance and is membranous in nature. In keeping with its vesicular appearance is the fact that light scattering by this preparation shows marked changes as the molarity of the suspending medium is altered. Treatment of this preparation with 0.5 per cent deoxycholate solutions removes a large part of the lipide material, which comprises almost half of the dry weight of the preparation. The residue, which still contains the "core" of the cytochrome electron transmitter system, as shown by spectroscopic and enzymatic experiments, is still structured and is membranous in morphological appearance.
It is concluded that the enzyme preparation is largely composed of fragmented mitochondrial membranes, and some of the consequences of the localization of the succinate and DPNH oxidase systems in or on these membranes are discussed.