Photodamage to lens hexokinase has been investigated by exposing the lenses of rat, rabbit and calf eyes to 300 nm irradiation. Hexokinase activity was diminished by 15.9%±5.4 and 23.4%±5.0 upon irradiation of the isolated rat lens for 1 and 2 hours respectively. Irradiation of the whole eye for 2 hours resulted in hexokinase deactivation of 13.6%±5.8 and 19.2%±6,2 for rat and rabbit lens homogenates and 55%±7 for calf lens capsule plus epithelium. Enzyme deactivation was prevented when the isolated lens was irradiated with the vitreous attached. Glucose, catalase or ascorbate added to the medium prior to irradiation, each had a protective effect on hexokinase deactivation. The results are consistent with a mechanism in which photochemical generation of active species of oxygen, via the photosensitizing action of tryptophan photoproducts, plays a significant role in enzyme deactivation.