The effect of amphotericin B (AMPH) on vacuolation in the budding yeasts Saccharomyces cerevisiae and Candida albicans was studied. The minimum inhibitory concentration of AMPH for growth of S. cerevisiae and C, aibicans was 1μg/ml. In untreated control cultures, mature cells had large central vacuoles in the exponential phase, which hampered the detection of vacuolation effect. Small buds in untreated exponential phase cells, however, only rarely showed vacuoles under the light microscope. Treatment with 0.2μg/ml of AMPH for 20-30min induced extensive vacuolation not only in mothers but also buds of S. cerevisiae. Extensive vacuolation lasted 4h or more, and growth rate of the cells was much reduced for 8h or more. Vacuolation itself was not fatal: on removal of the drug most cells gradually recovered from vacuolation and eventually multiplied. A similar effect of AMPH was also observed in C. albicans but at a higher concentration (0.5μg/ml).