Catalase is a key enzyme involved in the hydrogen peroxide metabolism under oxygen stress conditions. C-262T polymorphism of the catalase gene is linked to a higher risk of certain diseases. In this paper, the viability of mononuclear leukocytes with different variants of C-262T polymorphism in the catalase gene (CC, CT, TT) was examined under oxidative stress induced by hydrogen peroxide. It was found that cells with TT polymorphism are less viable at the 1 mM of hydrogen peroxide than are cells with CC and CT genotypes. Unlike CC and CT variants, the mitochondrial pathway of apoptosis was not activated in cells with TT polymorphism even with 1 mM hydrogen. These results may explain the higher risk of diseases associated with an antioxidant system in patients with C-262T promoter polymorphism of the catalase gene.