AbstractSome slow‐folding phases in the in vitro refolding of proteins originate from the isomerization of prolyl‐peptide bonds, which can be accelerated by a class of enzymes called prolyl isomerases (PPIs). We used the in vitro folding of an antibody Fab fragment as a model system to study the effect of PPI on a folding reaction that is only partially reversible. We show here that members of both subclasses of PPIs, cyclophilin and FK 506 binding protein (FKBP), accelerate the refolding process and increase the yield of correctly folded molecules. An acceleration of folding was not observed in the presence of the specific inhibitor cyclosporin A, but still the yield of correctly folded molecules was increased. Bovine serum albumin (BSA) increased the yield comparable to cyclophilin but, in contrast, did not influence the rate of reactivation. These effects were observed only when cyclophilin or BSA were present during the first few seconds of refolding. However, the rate‐limiting reactivation reaction is still accelerated when PPI is added several minutes after starting refolding. In contrast, the prokaryotic chaperone GroEL influences the refolding yield when added several minutes after initiating refolding. The results show that PPIs influence the folding of Fab in two different ways. (1) They act as true catalysts of protein folding by accelerating the rate‐limiting isomerization of Xaa‐Pro peptide bonds. Proline isomerization is obviously a late folding step and has no influence on the formation of aggregates within the first seconds of the refolding reaction. (2) PPI and BSA are able to increase the yield of refolding of Fab by reducing the formation of aggregates or the adsorption to the surface of the reaction vessel in an unspecific manner. This behavior is clearly distinct from the mechanism of action observed with the chaperone GroEL.