DsbC is a soluble protein of the bacterial periplasm that was identified genetically as being involved in protein disulfide formation. The gene sequence was corrected to include an additional proline residue and was then consistent with the molecular weight of the purified protein. Gel filtration and subunit hybridization indicate that DsbC is a stable dimer of identical subunits. Each subunit has a -Cys-Gly-Tyr-Cys- segment that forms an unstable and reactive disulfide bond; only the first cysteine residue is accessible, similar to thioredoxin and DsbA. The other two cysteine residues of DsbC form a buried, structural disulfide bond. The reactivities and stabilities of the active site disulfide bond of DsbC have been characterized and compared to that of DsbA. Both are very unstable and can be transferred rapidly to reduced proteins and peptides, although they differ somewhat in their kinetic reactivities. The two active sites of the DsbC dimer appear to function independently. DsbC is much more active than DsbA in catalyzing protein disulfide rearrangements, and this may be its main function in vivo.