The ability of Escherichia coli thioredoxin and thioredoxin reductase was examined with respect to protect the cells against hydrogen peroxide (H_2O_2) and super-oxide-generating agents. In late stationary phase, thioredoxin-and thioredoxin reductase-deficient mutant, trxA and trxB,respectively, exhibited increased sensitivity to these oxidizing agents compared with wild-type strain. Exponentially growing cells of trxA mutant was also more sensitive to H_2O_2 than wild-type strain. On the other hand, trxB mutant in exponentially growing phase acquired resistance to H_2O_2 over the level of wild-type strain. Catalase-hydroperoxidase I (HPI), encoded by the katG,is known to be the major defense enzyme against peroxides and inducible by H_2O_2. We examined the level of HPI expression in trxA and trxB mutants after treatment with H_2O_2. Interestingly, the trxB mutation stimulated the expression of katG : : lacZ fusion gene, while the trxA reduced. These results suggested that thioredoxin protects E.coli cells against H_2O_2 by at least two different mechanisms, by serving as a radical scavenger in its reduced form and by stimulating the induction of HPI in its oxidized form.