Cytochrome c binds certain physiological anions that are known to modulate the biological properties of the protein, although it is not known whether this effect is fortuitous or has physiological significance. We have examined the ability of the protein and its semisynthetic analogues to associate with certain of these anions, e.g. ATP, ADP, Pi and citrate. Our results show that specific residues or clusters of residues on the surface of horse heart cytochrome c are involved in the recognition sites for these anions. We also observed that binding at one site is linked to the oxidation state of the protein.