Purpose. A Class 3 aldehyde dehydrogenase happens to be a major soluble protein constituent of the cornea. Its role is conjectured to be manifold: to protect the tissue from oxidative damage by eliminating the toxic aldehydes produced upon lipid perox-idation under oxidative stress, to act as an UV-absorber, and to maintain the level of the coenzyme NADH in the cornea. We have studied the effect of UVB on the structure and enzyme activity of corneal aldehyde dehydrogenase.Methods. Aldehyde dehydrogenase was irradiated at 295 nm for varying periods of time and change in its enzyme activity assayed. The structural changes in the molecule accompanying irradiation were monitored using fluorescence and circular dichroism spectroscopy, and its hydrodynamic behavior and surface hydrophobicity studied using gel filtration chromatog-raphy and binding of the hydrophobic fluorophore ANS. The protective ability of aldehyde dehydrogenase in preventing aggregation of photolabile proteins, such asΓ-crystallin of the eye lens, was studied by monitoring the scattering value of the test protein with irradiation by UVB.Results. Aldehyde dehydrogenase is seen to undergo photo-damage with alterations in its quaternary structure, though no significant change is noticed in the peptide chain conformation. Under such conditions the molecule continues to act as a protec-tant by preventing aggregation of photolabile proteins such as the eye lensΓ-crystallin.Conclusions. Our earlier studies have shown that the free sulfhy-dryl groups are important for the antioxidant abilities of aldehyde dehydrogenase. Its protective ability towards photoaggregation ofΓ-crystallin seen here might arise both due to: (i) oxyradical quenching and (ii) the increased surface hydrophobicity of the molecule upon irradiation, which allows it to bind to, and thus inhibit the aggregation of interacting proteins.