The photooxidation of calfγ-crystallin has been Investigated in (a) riboflavin-sensitized reaction and (b) direct photolysis of the tryptophan residues of the protein at 300 nm. Partial insolubilization of the protein is evident by the increase in turbidity of the photolyzed protein solution. The turbidity is diminished when anaerobic conditions are used for irradiation. Spectroscopic studies on the soluble phase of the photolyzed protein reveal significant changes in the near-UV circular dichroism spectrum, suggesting that changes in the tertiary structure of the protein precede insolubilization. Isoelectric focusing analyses reveal that theγ-crystallin polypeptides have more acidic pI's after photooxidation under both conditions. The insoluble protein is highly crosslinked into dimers and higher oligomers via covalent, non-disulfide crosslinks, but inter-subunit crosslinking is negligible in the soluble phase of the photolyzed protein. The mechanism of photooxidation involves the generation of H2O2in the riboflavin-sensitized reaction. In the direct photolysis of tryptophan residues by 300 nm irradiation, there Is a rapid disruption of the protein structure, apparently by the oxidation of the hydrophobic tryptophan residues.