Protein disulfide isomerase (PDI) is a multifunctional protein catalysing the formation of disulfide bonds, acting as a molecular chaperone and being a component of the enzymes prolyl 4-hydroxylase (P4H) and microsomal triglyceride transfer protein. The role of PDI as a molecular chaperone or polypeptide-binding protein is mediated primarily through an interaction of substrates with its b' domain. It has been suggested that this binding is regulated by the redox state of PDI, with association requiring the presence of glutathione, and dissociation the presence of glutathione disulfide. To determine whether this is the case, we investigated the ability of PDI to bind to a folding polypeptide chain within a functionally intact endoplasmic reticulum and to be dissociated from the alpha-subunit of P4H in vitro in the presence of reducing or oxidizing agents. Our results clearly demonstrate that binding of PDI to these polypeptides is not regulated by its redox state. We also demonstrate that the dissociation of PDI from substrates observed in the presence of glutathione disulfide can be explained by competition for the peptide-binding site on PDI.