A mitochondrial preparation containing the complete electron transfer chain (EP1) as well as a preparation containing only one sector of the chain (Complex IV) were resolved into structural and catalytic protein fractions. The respective oxidation-reduction components of EP1 and Complex IV, as well as the lipid, were concentrated exclusively in the catalytic protein fraction. Integrated electron transfer activity appears to be unaffected by the removal of structural protein. Such losses in electron transfer activity as are sustained appear to be referable to the severity of the procedure for resolution rather than to the removal of structural protein. Under appropriate conditions structural protein will recombine with catalytic protein to “reconstitute” a particle that resembles the parent particle in respect to solubility, activity, spectral properties, and ultrastructure. The membrane-forming capability of the complexes is not affected by the removal of structural protein.