The formation of disulfide bridges in membrane and secretory proteins occurs during the protein folding process in the endoplasmic reticulum of eukaryotic cells and the periplasm of prokaryotes. The formation of disulfide bridges is facilitated by the thiol-disulfide oxidoreductases, protein disulfide isomerase (PDI) in eukaryotes and dsbA in prokaryotes. Structure-function analysis of these soluble proteins demonstrates that their active sites are sequences with similarity to the active site of the redox protein thioredoxin. Although these active sites share homology with thioredoxin, it is evident that other structural determinants change the redox characteristics of these proteins to enable them to facilitate the formation of correct disulfide bridging in the nascent protein. The analysis of structure-function relationships of PDI and dsbA have indicated that these thiol-disulfide oxidoreductases act as protein oxidants to facilitate the formation of disulfides during the folding process. The ability of PDI and dsbA to catalyze the formation and/or isomerization of disulfide bridging establishes their usefulness in facilitating the in vitro and in vivo folding of proteins. Protocols for the purification and assay of PDI activity have been described. Systems for the expression of PDI in Escherichia coli and Spodoptera frugiperda cells have been developed which may prove useful in the expression of recombinant proteins.