Loss of viability of toxin-treated cells of Saccharomyces cerevisiae SCF 1717 could be prevented in the period before they altered physiologically if cells were incubated in media with a suitable concentration of potassium (0.08 to 0.13 M) and hydrogen ions (pH 6.2 to 6.7). Incorporation of higher amounts of potassium chloride in the media had a pronounced negative effect on cell survival, particularly when the pH of the medium was lowered. Replacement The killer toxin produced by Pichia kluyveri 1002 affects the permeability of the plasma membrane in susceptible cells ofSaccharomyces cerevisiae after a treatment of about 60 min (6a; E. J. Middelbeek, Antonie van Leeuwenhoek J. Microbiol. Serol., in press). This change in permeability results in a decrease of intracellular pH, leakage of cellular potassium, and inhibition of energy-dependent processes, such as the uptake of amino acids (6a). Energy is required for the initiation of irreversible toxin action in susceptible cells (Middelbeek, in press). In this respect, the P. kluyveri toxin and the killer toxin excreted by S. cerevisiae K-12 (10) resemble various bacteriocins (3,12 Preparation ofthe killer toxin and toxin assay. Cells of P. kluyveri 1002 were grown, and 100-fold concentrated solutions of the toxin were prepared as described previously (6a). After dialysis against 0.05 M succinic acid-tris(hydroxymethyl)aminomethane buffer, pH 4.3, the concentrated sample was stored at -200C.Toxin activity was assayed and expressed in arbitrary units as reported previously (6). The titer of the samples used throughout this study was 2,000 to 3,000 arbitrary units per ml.Conditions