Auxins, like most plant hormones are involved in many developmental processes. Their mode of action is only weakly understood, however, and may involve cytoplasmic as well as membraneous or nuclear steps (Napier and Venis, 1990). Recent evidence indicates, at least in the case of cell elongation, auxins may be active at the outside of cells (Veniset al.1990). This suggests that primary events take place at the plasma membrane. The hypothesis of membrane-located effects has been thoroughly investigated in the past, particularly with regard to auxin-induced proton extrusion (Cleland, 1988). More recently it was shown, on tobacco protoplasts, that auxin promotes a hyperpolarization of the plasma membrane which is controlled by auxin-binding protein(s) (Barbier-Brygooet al.1990). All these data suggest that the electrogenic H+-ATPase of the plasma membrane is involved in the cellular response to auxin. However, almost no direct information is available on the sensitivity of the plasma membrane H+-ATPase to auxin, as most data have been obtained with quite crude membrane fractions (Scherer, 1984; Gabathuler and Cleland, 1985).