A number of mutant forms of horse cytochrome-c with single or double substitutions of lysine residues near the heme cavity was prepared that provided an interaction of mitochondrial ubiquinone with cytochrome-c reductase (EC 1.10.2.2) (complex III) and cytochrome-c oxidase (EC 1.9.3.1) (complex IV). The succinate cytochrome-c reductase and cytochrome-c oxidase activities of mitoplasts of rat liver were measured in the presence of mutant forms of cytochrome-c. The lysine residues in positions 8, 27, 72, 86, and 87 were shown to be the main contribution to the formation of a reactive complex with ubiquinol-cytochrome-c reductase of the respiratory chain, whereas the lysine residues in positions 13, 79, 86, and 87 were predominantly responsible for the formation of a complex with cytochrome-c oxidase.