Saccharomyces cerevisiae cells grown either aerobically or anaerobically were tested for tolerance to a brief heat stress (52°C, 5 min) or oxidative stress (20 mM H2O2, 15 min). Tolerance was related to growth phase, in that stationary phase cells were intrinsically more resistant to heat or oxidative stress than exponential phase cells. A mild heat shock (37 °C, 30 min) induced thermotolerance and oxidative tolerance in both aerobic and anaerobic cells. However, prior exposure to a low concentration of H2O2 (0.1 mM, 60 min) induced protection against the lethal concentration of H2O2 but not against the lethal temperature. Sensitivity to both heat and oxidative stress was dependent on membrane lipid composition. In the case of anaerobic cells, the most stress resistant had membranes enriched in saturated fatty acids, followed in order by cells enriched in oleic and linolenic acids. Aerobic cells with membranes enriched in palmitoleic and oleic acids showed the highest resistance to stress under all conditions. In both aerobic and anaerobic cells, a mild heat shock or oxidative shock induced markedly increased levels of thiobarbituric acid reactive substance (TBARS), indicative of malondialdehyde formation and lipid damage. Anaerobic cells with membranes enriched in linolenic acid had the highest TBARS, followed by cells enriched in oleic acid, with cells enriched in saturated fatty acids showing the lowest TBARS. The results suggest that heat and oxidative stress may share a common mechanism of damage through induction of oxygen-derived free radicals, resulting in membrane lipid damage. The extent of cellular damage was related to membrane lipid composition and correlated positively with increasing unsaturation of the phospholipid fatty acyl component.