Cytochrome c oxidase subunit VIIc was isolated from porcine intestine. Its amino acid sequence, starting at position 17 of the predicted precursor peptide, differs at eight positions from the human form, two positions from the bovine and at one from the mouse form. Although the peptide does not contain cysteine, it was bound specifically to thiopropyl-Sepharose 6B. The developed method makes it possible to obtain this protein in high purity and large quantities for studies of its putative modulatory role in the catalytic actions of the whole enzyme complex.