Thioredoxin-like protein (TlpA) is a membrane-anchored periplasmic protein that contains a thioredoxin domain at its C-terminus. An Agrobacterium tumefaciens mutant lacking functional tlpA shows increased sensitivity to a superoxide generator, increased resistance to H2O2, and reduced cytochrome c oxidase activity. Whereas the levels of antioxidant enzymes, including total superoxide dismutase (SOD) and catalases, are unaltered, high expression of periplasmic SOD partially restores the superoxide hypersensitivity of the mutant, suggesting an accumulation of superoxide anions in the periplasm. The change in the ability of the mutant to cope with H2O2 stress is independent of OxyR, an H2O2 sensor and transcription regulator. The presented data indicate that TlpA not only is involved in the proper assembly of cytochrome c but is also implicated in the bacterial oxidative stress response.