When yeast cells are grown in low but inhibitory concentrations of ethanol, the proton-pumping activity is stimulated but no significant increase of the plasma membrane permeability is observed These cell responses can only be appraised through the effect of ethanol on the acidification curves when cells have been grown in the presence of ethanol. In addition, and since the in vivo activation of plasma membrane H+ -ATPase by ethanol is rapidly reversed in vivo after its removal, yeast cells to be used to obtain the acidification curves should be maintained in permanent contact with ethanol. However; when the differently ethanol-tolerant strains Saccharomyces cerevisiae IGC 3507 III and Kluyveromyces marxianus IGC 2671 are grown in the presence of concentrations close to the maximal for growth, the development of responses to counteract the dissipation of the proton motive force induced by ethanol is hindered. Therefore, for these high concentrations the ethanol-induced increase of the final external pH attained after glucose addition in aqueous suspensions of deenergized cells, grown in the absence of ethanol, can be used as a rapid criterion for the evaluation of ethanol tolerance.