Abstract To elucidate the organization of cytochrome oxidase in the mitochondrial membrane, a study has been made on the correlation between the fine structure of a cytochrome oxidase-rich submitochondrial membrane (green membrane) and purified cytochrome oxidase. The green membrane was obtained directly from beef heart mitochondria or from the mitochondrial electron-transfer particles by treatment with deoxycholate and potassium chloride, followed by centrifugation. Heme a content of the green membrane isolated from beef heart mitochondria and from the electron-transfer particles were about 2 nmoles per milligram protein and 3–4 nmoles per milligram protein, respectively. Electron micrographs of the green membrane revealed regular arrays of small particles on the surface of the membrane, each particle measuring approximately 50–60 Å in diameter with a center-to-center distance of about 70 Å. These particles were extracted from the green membranes and recovered in a purified cytochrome oxidase fraction. Membrane structure was re-formed with the purified cytochrome oxidase, and the surface of the reconstituted membrane exhibited a particulate structure similar to the original green membrane. These results indicate that there is a correlation between the particles on the green membrane and cytochrome oxidase.