The polypeptide subunits of cytochrome c oxidase (E.C. 22.214.171.124.) isolated from beef heart which react with 203[Hg]Cl2 have been identified. A rapid and simple method for identifying and numbering the subunits is described which is independent of the type of sodium dodecyl sulphate-polyacrylamide gel system employed to separate them. © 1981.