1. As shown already by Orii and Okunuki (l), cytochrome oxidase**[EC 1.9.3.1] prepared by the method devised by Okunuki el al. (2) existed in an aggregated state. Enzymatic activity of the oxidase was activated by sodium dodecyl sulfate aat low concentration as a result of depolymerization of the original enzyme with s20, w of 22– 23 –3 to the 16–17-S and 6–7-S components.The 16–17-S component had much higher specific activity than that of the original one, but further activation was not observed by additional action of SDS. PCMB facilitated dissociation of the 16–17-S (dimer per heme a )component to the 6–7-S one in the presence of SDS and simultanously removed the copper from the oxidase. These results are compatible with the view that the functional unit of the oxidase contains two heme a molecules in it in aan unseparable fashion. 2. Under alkaline conditions the presence of rapidly reducible one, indicatting that the 830-mμ band was independent of the chromophore(s) of the CO-binding component(3, 4) 3. From the EPR studies carried out at 20°K, it became apparent that the enzyme contained two sorts of low spin hemes a (gx=1.55, gy=2, gz=3.01 and gx=1.88 gy=2.26, gz=2.54)in fairly comparable amounts and a few amount of high spin heme a (ca. 20% of the total heme a). In the oxygenated enzyme, most of the essential copper and a considerable amount of low spin heme a with g=3.01 were in oxidzed state. The result showed that change of the copper valence was synchronized with the redox- behavior of the 830-mμ band, and that reduced copper was more easily oxidized by aeration than the ferrous heme a component with g=3.01. 4. Measurement of the paramagnetic susceptibility from 273°K to 77°k revealed that there was no thermal change in the spin state of heme a of cytttochrome oxidase contrary to the suggestion by Ehrenberg and Yonetani (5)), so that if one low spin heme a is exchangeable to the high spin state, as shown by van Gelder el al. (6), the exchanging mechanism should be sought in other facrors such as conformational changes in the enzyme molecule.