Reducing and protein-refolding activities of adult T cell leukemia-derived factor (ADF)/human thioredoxin were studied. Recombinant ADF/human thioredoxin produced by E. coli, which has an insulin-reducing activity as efficient as that of E. coli thioredoxin, also reduced some reactive oxygen species, such as hydrogen peroxide. Furthermore, recombinant ADF/human thioredoxin was found to have protein-refolding activity for scrambled (mispaired disulfide-containing) RNase A. Cys-31 at the active site of ADF/human thioredoxin proved essential for reducing activity, and loss of Cys-31 in ADF/human thioredoxin attenuated the protein-refolding activity. These data suggest a physiological role of ADF/human thioredoxin in protecting living cells from proteotoxicity caused by reactive oxygens in vivo.