Monoclonal antibodies reacting with the inhibitor protein (IF1) of the mitochondrial ATPase/ATP synthase complex did not modify the IF1‐induced inhibition of soluble F1 ATPase activity. On the contrary, they increased the ATPase activity of inverted electron‐transport particles without inducing a significant release of IF1 from these particles. This suggested that IF1 could be linked to a membrane protein when it was not inhibiting the ATPase activity. IF1 antibodies have been used to show that IF1 can bind not only to the β subunit of F1‐ATPase [Klein, G., Satre, M., Dianoux, A. C. & Vignais, P. V. (1981) Biochemistry 20, 1339–1344] but also to a protein present in the inner‐mitochondrial membrane. The cross‐linking of IF1 to this membrane protein gave a product of Mr 15000–16000 that migrated differently from IF1 and from the dimer of IF1 using SDS/PAGE. When the cross‐linked product was obtained by using a cleavable cross‐linking reagent, the complex between IF1 and the docking protein was partly dissociated and free IF1 was recovered. Considering the molecular mass of IF1, this docking protein for IF1 has apparent Mr 5000–6000. The complex between IF1 and this receptor protein can be detected in low amounts by antibodies against IF1 in the absence of cross‐linking reagent. Since this complex remained in the pellet after treatment of the membrane with Triton X‐100, it should be a membrane protein. Therefore, IF1 can bind not only to its inhibitory‐binding site at the β subunit of F1, but also to a non inhibitory site which is a membrane protein of approximate Mr 5000–6000.