The effect of UV irradiation on bovine lens soluble proteins (crystallins) in the presence of tryptophan metabolites was investigated in vitro. The cross-linking of crystallins by UV irradiation was accelerated by kynurenine, 3-hydroxykynurenine, anthranilic acid and 3-hydroxyanthranilic acid. On denaturation of crystallins by photooxidation, α-crystal-lin was characterized by the formation of water-soluble HMW (high molecular weight) protein, while water-insoluble HMW protein was produced from β- and γ-crystallin. These HMW aggregates showed cross-linking by non-disulfide covalent bonds. LMW (low molecular weight) peptides were formed by degradation of α- and β-crystallin.