We have demonstrated that Helminthosporium maydis, race T, toxic culture filtrates induce electrolyte leakage from treated susceptible corn (Zea mays L.) leaf cells. In a comparison of several cytoplasmic versions of the inbred W182BN, those versions susceptible to the fungus and its toxin in the field demonstrated induced electrolyte leakage while versions rated resistant in the field did not. The total amount of electrolyte leakage of leaves from susceptible Texas cytoplasmic male sterile plants increased with increasing toxin concentrations. Leaves from non-male sterile and resistant cytoplasmic male sterile plants, although much more resistant than T cytoplasm leaves, were not immune to the toxin. If the toxic filtrate concentration or the treatment duration were greatly increased, leaves of the resistant plants exhibited some electrolyte leakage. The electrolyte leakage suggests damage to cellular membranes. This damage may be the result of mitochondrial inactivation induced by the toxin, or it may result from the effect of the toxin on other cellular sites.