Certain ocular proteins have been found to be chemically modified by exposure to near-UV light (320-390 nm) in the presence of tryptophan. Colored and fluorescent tryptophan photoproducts bind firmly to proteins, thereby altering their physico-chemical properties. The question of whether such a reaction would inhibit the catalytic action of catalase is herein raised. When solutions of bovine liver catalase were re-incubated up to 24 hr under near-UV with preirradiated tryptophan and dialyzed, most of the ability of the enzyme to decompose H2O2 was lost. Similar results occurred for catalase activities of bovine cornea and lens epithelia. The enzyme protein exhibited altered UV absorption and fluorescence spectra and increased electrophoretic mobility after binding photoproducts, Near-UV light photoproducts of tryptophan are thus capable of deactivating crystalline and tissue catalase.