doxycycline gradually re- duces them to 20% of wild-type. Under oxidative-stress conditions, the progressive decrease in Hsp60 levels in these mutants resulted in reduced cell viability and an increase in both cell peroxide species and protein car- bonyl content. Protection of Fe/S-containing enzymes from oxidative inactivation was found to be dose-de- pendent with respect to Hsp60 levels. As these enzymes release their iron ions under oxidative-stress condi- tions, the intracellular labile iron pool, monitored with calcein, was higher in cells with reduced Hsp60 levels. Consistently, the iron chelator deferoxamine protected low Hsp60-expressing cells from both oxidant-induced death and protein oxidation. These results indicate that the role of Hsp60 in oxidative-stress defense is explained by protection of several Fe/S proteins, which prevent the release of iron ions and thereby avert further damage.