Cytochrome c oxidases, purified from bovine heart and the thermophilic bacterium PS3, were irradiated with a high‐energy electron beam. The proton transport activities of both preparations and their electron transfer activities decreased as single exponential functions of the radiation dosage. Applying the target theory with alkaline phosphatase as an internal standard, the following functional molecular weights were obtained for cytochrome c oxidation and H+ pumping: 63‐73 kd and 160‐220 kd, respectively, for the bovine enzyme, and 80‐100 kd and 190‐230 kd for the PS3 enzyme. The results suggest that a dimer structure is necessary for H+ pumping, while a core part of monomer (presumably the largest two subunits, i.e. subunits I and II) is sufficient for cytochrome c oxidation.