Plasma membrane-enriched vesicles were isolated by density gradient centrifugation from roots of zinc-sufficient and zinc-deficient bean (Phaseolus vulgaris L. cv. Prelude) plants. The two populations of vesicles had similar activities of specific membrane marker enzymes and ATP hydrolysis and were competent for proton transport. However, vesicles from zinc-deficient roots showed lower rates of ATP-dependent intravesicular acidification and increases in passive permeability to protons as well as in the rate of dissipation of a non-metabolic transmembrane pH gradient. The decrease of the rate of proton accumulation in isolated vesicles closely paralleled the increase in potassium leakage from intact roots and the appearance of visual zinc deficiency symptoms in the shoots. Re-supply of zinc to deficient plants for 24 h promoted shoot growth, reduced potassium leakage from roots and led to partial recovery of the proton accumulation capacity and to a decrease in passive permeability to protons in isolated vesicles. The results obtained with isolated vesicles confirm the previously observed 'in vivo' effects of zinc deficiency and are consistent with the idea that an alteration of plasma membrane lipids leads to an increase in permeability and an impairment in trans-plasma membrane proton gradient.