Mg2+ at 3.5 mM increased the tolerance of a self-flocculating fusant of Schizosaccharomyces pombe and Saccharomyces cerevisiae to ethanol. After 9 h of exposure to 20% ( v/v) ethanol at 30 degreesC, all cells died whereas over 50% remained viable for the cells grown with Mg2+. The effect of Mg2+ is closely related to its ability to decrease plasma membrane permeability of cells subjected to ethanol stress.