Cytochrome c oxidase in detergent dispersion has been cross-linked with two reversible cross-linking agents, dithiobissuccinimidylpropionate (DSP) and dimethyl-3,3'-dithiobispropionimidate (DTBP), and the cross-linked products formed have been analyzed by two-dimensional gel electrophoresis. Under mild reaction conditions, several subunit pairs were seen including II and V, V and VII, IV and VI. With higher levels of DSP, larger aggregates were seen until a cross-linked product with an apparent molecular weight of 140 000 was the predominant band on gels. This is the smallest molecular weight aggregate to contain all seven subunits of the enzyme and most likely represents the "unit" or two heme and two copper containing complex of cytochrome c oxidase.