Protein disulfide bond formation in Escherichia coli requires the periplasmic protein DsbA. We describe here mutations in the gene for a second protein, DsbB, which is also necessary for disulfide bond formation. Evidence suggests that DsbB may act by reoxidizing DsbA, thereby regenerating its ability to donate its disulfide bond to target proteins. We propose that DsbB, an integral membrane protein, may be involved in transducing redox potential across the cytoplasmic membrane.