Cytochrome c oxidase from bovine heart was dissociated into its protein subunits by sodium dodecylsulphate, the subunits were separated on a preparative scale by sodium dodecylsulphate gel permeation chromatography. The subunits elute upon gel chromatography in order of decreasing apparent relative molecular mass (I, 40000; II, 26000; III, 21000; IV, 17000; V and VI, 12000; VII, 10000 and VIII, 6000). The very hydrophobic subunits I and III tend to form small aggregates both in the presence and absence of sodium dodecylsulphate. The molar ratio of the subunits was determined by two methods: firstly by quantitative amino acid analysis of each subunit peak, and secondly from the absorbance of each subunit at 280 nm caused by tryptophan and tyrosine. We conclude that the subunits I to VI are present in 1 : 1 ratio; our fraction VII contains two stoichiometric polypeptides which may or may not be identical. Fraction VIII contains enough protein for four stoichiometric chains which may belong to three different types. The 12 stoichiometric chains add up to a Mr of about 170000 if the sizes of I and III are 40000 and 21000, respectively. After correction for the presence of aggregates, subunits I and III appear to be present in more than 1 : 1 stoichiometric amounts with respect to other subunits, which probably means that subunits I and III are considerably larger than hitherto assumed. This is in line with recently published mtDNA sequence work [Anderson, S. et al. (1981) Nature, 290, 457--465].