The lysosomotropic detergent N-dodecylimidazole (C12-Im) has previously been found to kill mammalian cells by concentrating in lysosomes, followed by lysosomal disruption and release of cytotoxic enzymes into the cytoplasm. The action of C12-Im on Saccharomyces cerevisiae is described in this report. C12-Im prevented growth of colonies when present in 1% yeast extract-2% Bacto-Peptone-2% glucose plates at concentrations of 5 micrograms/ml or above, or when present in a soft agar overlay at 20 micrograms/ml. Treatment of cells suspended in glucose-containing buffer (pH 8.0, 37 degrees C) with C12-Im (6 micrograms/ml) caused greater than 95% cell death within 6 min. Dependence of killing on C12-Im concentration was sigmoidal, suggesting a cooperative mode of action. Killing was pH dependent, being much more effective at pH 8.0 than at pH 5.0. Ammonium sulfate and imidazole protected against killing if added before, but not after, the addition of C12-Im. Sensitivity to C12-Im was strongly growth dependent: the cells were most sensitive at early to mid-logarithmic phase of growth and became progressively less sensitive during progression through late logarithmic and stationary phase. Vacuolar disruption by C12-Im was demonstrated by using cells loaded with lucifer yellow CH or fluoresceinated dextran in their vacuoles; vacuoles of logarithmically growing cells were more sensitive than those of stationary-phase cells. These results suggest that vacuolar disruption by C12-Im may underlie its cytotoxic effects.