Bone resorption depends on the formation, by osteoclasts, of an acidic extracellular compartment wherein matrix is degraded. The mechanism by which osteoclasts transport protons into that resorptive nicroenvironment was identified by means of adenosine triphosphate-dependent weak base accumulation in isolated osteoclast membrane vesicles, which exhibited substrate and inhibition properties characteristic of the vacuolar, electrogenic H +-transporting adenosine triphosphatase (H+-ATPase). Identity of the proton pump was confirmed by immunoblot of osteoclast membrane proteins probed with antibody to vacuolar H+-ATPase isolated from bovine kidney. The osteoclast's H+-ATPase was immunocytochemicaily localized to the cell-bone attachment site. Immunoelectron microscopy showed that the H +-ATPase was present in the ruffled membrane, the resorptive organ of the cell.