The hydrolytic activity of the ATP synthase in bovine mitochondria is inhibited by a protein called IF1, but bovine IF1 has no effect on the synthetic activity of the bovine enzyme in mitochondrial vesicles in the presence of a proton motive force. As part of an investigation into the inhibitory effects on the hydrolytic and synthetic activities of ATP synthase in bovine sub-mitochondrial particles, the absolute quantities of both the ATP synthase complex and the IF1 protein have been measured.