Beef heart cytochrome-c deficient Keilin-Hartree, pH 5.7, particulate manifests decreased succinate-ferricyanide reductase activity after lyophilization and pentane extraction. Addition of CoQ10 or pentane extract restores activity in a Michaelis-Menten fashion, maximum activity being ≊ 5 × greater than that of extracted particulate. Kinetic relationships, 1 v vs. 1 (S) and 1 v vs. 1/(ferricyanide) plots, for extracted particulate are different from those of untreated particulate and soluble succinate dehydrogenase. Addition of CoQ10 restores the kinetic relationships to those seen with untreated particulate. It is concluded that CoQ plays an essential role in the succinate-ferricyanide reductase activity of the particulate. © 1969.