Normal thymocytes and thymoma cells have been exposed to the toxic action of tert-butyl hydroperoxide (t-BOOH) to study the mechanisms of oxidative lethal cell injury and the sensitivity of normal and tumor cells to an oxidative stress. From our data, we can suggest that: (a) In thymoma cells two mechanisms can operate in determining t-BOOH toxicity: the first one, involving glutathione peroxidase and nicotinamide nucleotides, leads to cell death by alteration of calcium homoeostasis, and the second one, involving potent oxygen reactive species, affects cell viability by ATP depletion. The first mechanism is demonstrated by the decrease of nicotinamide nucleotides, the partial protection from lactate dehydrogenase (LDH) release by ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) (40%) and by dithiothreitol (DTT) (52%). The second mechanism is based on the partial protection of LDH release by deferoxamine (42%) and glucose (50%) and by total protection of ATP depletion by glucose. In agreement with these observations, DTT and deferoxamine in combination totally prevent cell death as well as EGTA and glucose do. (b) In normal thymocytes only the mechanism involving oxygen radical species is implicated. t-BOOH-induced death is not related to modifications of nicotinamide nucleotide pool. No changes are observed in LDH release by the addition of EGTA and DTT. In contrast, deferoxamine totally prevents cell death. Mechanisms depending on cellular ATP are involved in t-BOOH-induced oxidative damage. Glucose completely prevents t-BOOH-mediated ATP depletion and completely protects from LDH release. Our data indicate that tumor transformation deeply affects cell properties and metabolism, changing cell responsiveness to an oxidative stress.