The oxidative folding of bovine pancreatic trypsin inhibitor BPTI) has served as a paradigm for the folding of disulfide-containing proteins from their reduced form, as well as for protein folding in general. Many extracellular proteins and most pharmaceutically important proteins contain disulfide bonds. Under traditional conditions, 0.125 mM glutathione disulfide (GSSG) and no glutathione (GSH), the folding pathway of BPTI proceeds through a nonproductive route via N* (a two disulfide intermediate), or a productive route via N '(and other two disulfide intermediates which are in rapid equilibrium with N'. Both routes have the rearrangement of disulfide bonds as their rate-determining steps. However, the effects of the composition of the redox buffer, GSSG and GSH, on folding has not been extensively investigated. Interestingly, BPTI folds more efficiently in the presence of 5 mM GSSG and 5 mM GSH than it does under traditional conditions. These conditions, which are similar to those found in vivo, result in a doubly mixed disulfide between N ' and glutathione, which acts as an oxidative kinetic trap as it has no free thiols. However, with 5 mM GSSG and 5 mM GSH the formation of the double mixed disulfide is compensated for by N* being less kinetically stable and the more rapid conversion of the singly mixed disulfides between N ' and glutathione to native protein (N). Thus a major rate-determining step becomes the direct conversion of a singly mixed disulfide to N, a growth-type pathway. Balancing the formation of N* and its stability versus the formation of the doubly mixed disulfide and its stability results in more efficient folding. Such balancing acts may prove to be general for other disulfide-containing proteins.