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/mg of mitochondrial protein, and the affinity constant ($K_a$) to be $1.79{ imes}10^{7}M^{-1}$ 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.