We have used a polyclonal antibody probe to isolate cDNA clones for mouse cytochrome oxidase subunit Vb from a bone marrow tumor cell mRNA library in lambda gt11 expression vector. The mouse cDNA contains an open reading frame of 128 amino acids, which shows 81% positional identity with the predicted amino acid sequences of the human subunit. Northern blot analysis and sequencing of cDNA from a mouse kidney library show no tissue specific variations in subunit Vb.