alpha-Crystallin, a major lens protein, has many of the properties of a molecular chaperone, but its ability to assist refolding of proteins has been less certain. In the present work it was shown that alpha-crystallin specifically increased the reactivation of guanidine-denatured glyceraldehyde-3-phosphate dehydrogenase with most of the activity being recovered. In the incubation mixture the recovered enzyme activity was partly free but mostly it appeared in a protective complex with alpha-crystallin. The aggregation of the denatured enzyme on dilution from the guanidine solution was prevented. Thus alpha-crystallin not only protects against aggregation and inactivation of enzymes during denaturation, but can also prevent aggregation and assist recovery of the native structure during renaturation.