Monoclonal antibodies to subunits of bovine heart cytochrome c oxidase were prepared by immunizing mice with the isolated enzyme. The majority of antibody-producing cell lines were found to react with two different subunits of similar molecular mass, as shown by Western blotting and ELISA titrations with the HPLC-purified subunits. The affinities of the monoclonal antibodies to the subunits were determined by ELISA titrations with increasing concentrations of NH4SCN. Two monoclonal antibodies with a low affinity to subunit VIa had a high affinity to subunit VIc, whereas two other antibodies showed the same affinity to subunits VIIa and VIIb. The same affinity of monoclonal antibodies suggested an evolutionary relationship of subunits VIIa and VIIb, which was further supported by reactivity of these antibodies to subunits VIIa and VIIb of cytochrome c oxidase from different species and tissues. Also the evolutionary relationship between subunit VIa and VIc was shown by hybridization at low stringency of cDNAs for rat cytochrome c oxidase subunits VIc and VIa-h (heart-type), after amplification by the polymerase chain reaction, with a probe of VIa-l (liver-type).