The CD spectrum of oxidized cytochrome oxidase in the wavelength region 185-260 nm shows that the secondary structure of the protein consists of close to 60% alpha-helix and slightly less than 20% beta-structure. CD spectra of oxidized cytochrome oxidase, of half and fully reduced carboxycytochrome oxidase as well as of fully reduced cytochrome oxidase, have been recorded in the wavelength region 200-260 nm. The results demonstrate a conformational change on going from the oxidized to the half reduced state in carboxycytochrome oxidase; no further change occurs on full reduction. A conformational change is also seen in the fully reduced enzyme without bound CO. The conformational transitions are suggested to be part of the proton pumping mechanism of cytochrome oxidase.