Tumor necrosis factor-α (TNF-α) induces the activation of all three types of mitogen-activated protein kinase (MAPK): c-Jun NH 2-terminal kinase (JNK), p38, and extracellular signal-regulated kinase (ERK). This cytokine also induces the production of several types of reactive oxygen species, including H2O2. With the use both of HeLa cells expressing wild-type or dominant negative forms of the cytosolic peroxidase peroxiredoxin II and of mouse embryonic fibroblasts deficient in this protein, we evaluated the roles of H2O2 in the activation of MAPKs by TNF-α. In vitro kinase assays as well as immunoblot analysis with antibodies specific for activated MAPKs indicated that H2O2 produced in response to TNF-α potentiates the activation of JNK and p38 induced by this cytokine but inhibits that of ERK. Our results also suggest that cytosolic peroxiredoxins are important regulators of TNF signaling pathways.