Conformational changes in the three crystallins α β and γ in a singlet-oxygen generating system were investigated by fluorescence studies of tryptophan and covalently-bound sulfhydryl probe 4-[(N-iodoacetoxy)N-methyl]amino-7-nitrobenz-2-oxa-l,3-diazole (IANBD). Upon excitation at 295 nm, the tryptophan emission maxima of the crystallins were red-shifted by irradiation with visible light in the presence of the photosensitizer methylene blue. β crystallin showed the largest shift (A nm) of the emission spectrum. Time course of the fluorescence changes by irradiation showed that the decrease in the tryptophan fluorescence yield occurs most rapidly for βcrystallins, as compared to α or γcrystallins. Fluorescence changes of IANBD-labeled crystallins show a 40% decrease in the fluorescence intensity of the sulfhydryl probe for βcrystallin after one hour of irradiation. For α and γcrystallin smaller decreases (7% and 15% respectively) were observed. Since all the sulfhydryl groups of βcrystallin are known to be exposed on the surface of the protein (Andley et al, 1982, Biochemistry 21, 1853), these results suggest that the pronounced changes in conformation of βcrystallin by singlet oxygen may be due to a rapid loss of the protein tertiary structure by oxidation of the sulfhydryl groups. These results have potential significance in understanding the age and cataract-related changes in the ocular lens in view of the fact that several key lens enzymes are associated with βcrystallins in vivo.