The mechanism of H2O2-resistance of Hpr-4, a variant of Chinese hamster V79 cells, was investigated. The effect of H2O2on the mitochondria of the parental and Hpr-4 cells was compared. First, both biochemical and ultrastructural results showed that mitochondria in the parental cells were damaged by exposure to H2O2, while those in Hpr-4 cells recovered from the damage. Second, the H2O2resistance of Hpr-4 cells was reversibly reduced or recovered by the addition or removal of inhibitors of mitochondrial biosynthesis, respectively. Third, the parental cells were auxotrophic to pyruvate after exposure to H2O2. Fourth, H2O2-sensitivity of the parental cells was also enhanced by the inhibition of mitochondrial biosynthesis. From these results, it was concluded that the mitochondria of Hpr-4 cells apparently had a greater resistance to H2O2than those of the parental cells and that functional mitochondria were involved in the recovery of Chinese hamster V79 cells from H2O2-induced damage.