A resin glycoside mixture isolated from Ipomoea tricolor inhibited radicle growth of Echinochloa crusgalli. The effect of the resin was tested on the activity of the plasma membrane HC-ATPase from E. crusgalli. For this purpose, plasma membrane vesicles were purified by the method of aqueous two-phase partitioning. The resin glycoside inhibited by 30% the activity of the plasma membrane ATPase. The same result was obtained with the purified main component of the resin. This indicates that the plasma membrane ATPase can be one of the cellular targets of the resin. Hence it is possible that the mechanism of action of the resin involves an inhibition of the plasma membrane ATPase.