The endoplasmic reticulum resident oxidoreductase, protein disulfide isomerase (PDI), catalyzes the acquisition of native disulfide bonds in multi-disulfide-bond-containing proteins.The rate-determining step in the folding of such proteins generally involves a competition between thiol-disulfide exchange reactions and a conformational folding reaction.Using bovi(n)e pancreatic ribonuclease A (RNase A) as a model, we have examined the impact of PDI on the pivotal structure-coupled regeneration step in RNase A folding.Our results indicate that the isomerase activity of PDI is extremely detrimental to the conformational folding of native-like intermediates required for RNase A regeneration, though its chaperone-like activity does promote structure acquisition to a limited extent.Yet PDI accelerates the overall rate-determining step involving formation of RNase A native-like intermediates under conditions prevalent in the ER.A model that explains the maturation of ER-processed proteins in the light of conflieting PDI activity is presented.