Results are presented that confirm and extend earlier findings that efrapeptin is a potent inhibitor of oxidative phosphorylation. Binding of efrapeptin is shown to be reversible, and a dissociation constant for the enzyme-inhibitor complex is estimated to be 10(-8) M under conditions for either ATP synthesis or hydrolysis. Fifty per cent inhibition of the ATP hydrolysis activity of submitochondrial particles is obtained at a ratio of 0.56 mol of inhibitor/mol of enzyme. Studies of efrapeptin binding under pseudo-first order conditions show that the onset of inhibition is first order with respect to efrapeptin. Combined with the inhibition titer, these results indicate that there is one inhibitor binding site per molecule of enzyme. Steady state velocity studies using a substrate regenerating system show that efrapeptin is competitive with both ADP and phosphate during ATP synthesis. However, during ATP hydrolysis, a distinctly different mode of inhibition is indicated with respect to ATP. Data are presented which suggest that ATP promotes the binding of efrapeptin to the enzyme. Indications that efrapeptin is a catalytic site inhibitor make these results difficult to reconcile with a simple mechanistic scheme involving a single independnet catalytic site for ATP synthesis and hydrolysis. Our results are discussed in terms of support for catalytic cooperativity between adjacent subunits as recently proposed by Kayalar et al. (Kayalar, C., Rosing, J., and Boyer, P. D. (1977) J. Biol. Chem. 252, 2486-2491).