Binding of horse heart (methyl-^(14)C)cytochrome c reductively methylated with (^(14)C) formaldehyde to isolated rat liver mitochondria has been investigated. The number of binding sites is calculated to be 56 pmoles of cytochrome c/㎎ of mitochondria) protein, and the affinity constant (K_a) to be 1.79 × 107M^(-1). Various naturally occurring basic compounds including histones, protamine and polyamines are highly inhibitory on the (methyl-^(14)C)cytochrome c binding. Almost all of (methyl-^(14)C)cytochrome c bound to mitochondria can be released from the mitochondria by subsequent treatment with nonlabeled cytochrome c. Although histone H3 (arginine-rich histone) has much stronger inhibitory effect on the (methyl-^(14)C)cytochrome c binding than non-labeled cytochrome c at equimolar concentration when present in the binding assay mixture, only a fraction of bound (methyl-^(14)C)cytochrome c can be freed from mitochondria by treatment with histone H3. Evidence indicates that these effect are not merely a consequence of electrostatic influence on the cytochorme c receptor of mitochondria.