The exposure of dialyzed preparations of lens crystallins to copper (II) ions causes a decrease in protein surface thiol and the production of hydrogen peroxide (H2O2). H2O2 production by gamma and beta crystallin subfractions (which contain the greatest level of thiol) is the predominant source of this H2O2. Protein surface thiols are probable sources of H2O2 formation since N-ethyl maleimide treatment of lens proteins and zinc ions inhibit H2O2 production. These data are consistent with a hypothesis that transition metal-catalyzed oxidation of protein contributes to cataractogenic lens protein oxidations.