Plasmalemma from spinach leaves, a long day plant, has been isolated and characterized by biochemical markers. The in vitro effect of exogenous indole-3-acetic acid (IAA) on proton pumping activity was measured in plasmalemma from vegetative leaves (4 weeks in short day) or induced to flowering by different physical or chemical ways (continuous light, displaced short day, 8 weeks short days, gibberellic acid in short days). It appeared that the auxin activated the ATP-dependent accumulation of proton in plasma membrane vesicles. The activating effect of IAA was in nanomolar range in membranes from short day grown spinach. Plasma membrane from plants induced to flowering exhibited a second peak of auxin effect in the micromolar range.