Victorin, the pathotoxin produced by Helminthosporium victoriae, causes false plasmolysis and death of root cap cells, and this effect is selective for oat cultivars susceptible to the fungus. The development of a procedure for the isolation of root cap cells made possible a study of victorin's effects on populations of intact cells uniformly exposed to the toxin. Isolated root cap cells from resistant and susceptible cultivars were used to examine the effects of temperature and plasmolysis on victorin-induced cell death. Cell viability was determined by the ability to stain with fluorescein diacetate. Victorin-induced cell death was highly temperature dependent with a temperature coefficient of 3·5. At 10°C 96 to 98% of the cells remained alive for over 30 days when incubated in up to 50 units of toxin per ml. Plasmolysis also inhibited cell death; this protective effect was dependent upon solution tonicity. Non-permeating media, mannitol and sorbitol, protected more than sucrose in which cells slowly deplasmolyzed. Urea, a rapidly-permeating osmoticum, did not delay victorin-induced cell death. The modification of victorin's lethal effects by temperature and plasmolysis may explain variability in reported effects of the toxin on isolated protoplasts.