The rate and extent of uptake of the fluorescent probe diS-C3(3) reporting on membrane potential inS. cerevisiae is affected by the strain under study, cell-growth phase, starvation and by the concentration of glucose both in the growth medium and in the monitored cell suspension under non-growth conditions. Killer toxin K1 brings about changes in membrane potential. In all types of cells tested,viz. in glucose-supplied stationary or exponential cells of the killer-sensitive strain S6/1 or a conventional strain RXII, or in glucose-free exponential cells of both strains, both active and heat-inactivated toxin slow down the potential-dependent uptake of diS-C3(3) into the cells. This may reflect “clogging” of pores in the cell wall that hinders, but does not prevent, probe passage to the plasma membrane and its equilibration. The clogging effect of heat-inactivated toxin is stronger than that exerted by active toxin. In susceptible cells,i.e. in exponential-phase glucose-supplied cells of the sensitive strain S6/1, this phase of probe uptake retardation is followed by an irreversible red shift in probe fluorescence maximumλ max indicating damage to membrane integrity and cell permeabilization. A similar fast red shift inλ max signifying lethal cell damage was found in heat-killed or nystatin-treated cells.