Heparin inhibits transport of electrons from reduced cytochrome c to cytochrome c oxidase. The effect is due to the interaction of heparin with cytochrome c. It has been observed that binding of heparin to the reduced or oxidized cytochrome c changes the spectrum of cytochrome c at the Soret region. Affinity chromatography of heparin in cytochrome c immobilized to thiol-Sepharose shows that commercial heparin is eluted in the low-affinity and high-affinity fractions. Both participate in the interaction with cytochrome c. Polylysine induces decay of the cytochrome c-heparin complex.