Disulfide-containing proteins offer unique advantages for mechanistic studies of the formation of native three-dimensional structure from unordered, reduced precursors. The main advantage is that covalent intermediates are formed; by characterizing these intermediates, one obtains substantial information about the reaction pathway. Thiol-disulfide interchange is a major component of most oxidative mechanisms carrying thiol to disulfide; thus, it required some attention in its own right. Afinsen's descriptions of a "shuffle-ase" enzyme led us to examine the rates of the uncatalyzed exchange under physiologically plausible conditions. Somewhat surprisingly, we found that the rates for formation of several native proteins in uncatalyzed systems containing GSSG and GSH are as great as with the "shuffle-ase" enzyme, suggesting that a substantial portion of biological thiol oxidations proceed by uncatalyzed exchange. While thiol-disulfide exchange of course results in no net change in the oxidation level of a system, catalytic linkage of thiol or disulfide to other redox systems provides a mechanism for achieving net changes.