alpha -Crystallin, a major eye lens protein, has been shown to function like a molecular chaperone by suppressing the aggregation of other proteins induced by various stress conditions. Ultraviolet (UV) radiation is known to cause structural and functional alterations in the lens macromolecules. Earlier we observed that exposure of rat lens to in vitro UV radiation led to inactivation of many lens enzymes including glucose-6-phosphate dehydrogenase (G6PD). In the present paper, we show that alpha -crystallin (alphaA and alphaB) protects G6PD from UVB irradiation induced inactivation. While, at 25 degreesC, there was a time-dependent decrease in G6PD activity upon irradiation at 300 nm, at 40 degreesC there was a complete loss of activity within 30 min even without irradiation. The loss of activity of G6PD was prevented significantly, if alphaA- or alphaB-crystallin was present during irradiation. At 25 degreesC, alphaB-crystallin was slightly a better chaperone in protecting G6PD against UVB inactivation, Interestingly, at 40 degreesC, alphaA- and alphaB-crystallins not only prevent the loss of G6PD activity but also protect against UVB inactivation, However, alphaA- and alphaB-crystallins were equally efficient at 40 degreesC in protecting G6PD.