Eye Research Institute and Harvard Medical School, 20 Standford Street, Boston, Mass. 02114, U.S.A. Although the secondary structures of all lens crystallins are more or less similar, their tertiary structures are remarkably different as manifested in their near-uv circular dichroic and intrinsic or extrinsic fluorescence spectra. Methylene blue-sensitised photooxidation causes a change in the tertiary structure of lens crystallins but in each case subtle differences exist in the nature of the changes. Photoinduced changes in the conformation of lens protein are discussed in terms of destabilisation of the protein structure during cataractogenesis.