DsbC, which catalyzes disulfide isomerization, was overproduced in the periplasm of Escherichia coli and purified from the periplasmic fraction by osmotic shock and anion-exchange chromatography. The active site of the purified DsbC was found to be an oxidized form (ox-DsbC) which could be converted to the reduced form (red-DsbC) by the addition of dithiothreitol. The effect of ox- and red-DsbC on the refolding of chemically denatured and reduced proteins with different numbers of disulfide bonds and free cysteine-thiol groups was investigated. Ox-DsbC facilitated the refolding of proteins with multiple disulfide bonds in both oxidative and reductive environments, while red-DsbC facilitated refolding only in the former. On the other hand, only red-DsbC facilitated the refolding of proteins with multiple free cysteine-thiol groups but either form of DsbC did not facilitate the refolding of proteins with only one cysteine-thiol group. It is therefore important to choose the form which suits the properties of the protein. Holo-chaperonin from Thermus thermophilus and DsbC demonstrated a synergistic effect on protein refolding.