Active transport of catecholamines into chromaffin granules is driven by the transmembrane pH gradient and membrane potential, created by an electrogenic proton-translocating ATPase in the granule membrane. The ATPase activity of highly purified chromaffin granule membranes is inhibited by a number of agents in common with mitochondrial ATPase, and also by antibodies raised against mitochondrial F1. Dichloromethane treatment of these membranes solubilizes an enzyme that is closely similar to mitochondrial F1, but distinguishable from it by its interaction with specific antisera and the inhibitor aurovertin. Chromaffin granule membranes contain a low-molecular-weight protein that reacts with dicyclohexylcarbodiimide; it can be extracted into chloroform-methanol, and is of higher electrophoretic mobility than the corresponding mitochondrial protein. Evidence is presented that this is a component of the proton-translocating ATPase complex.