The present paper reports on the effects of UV-A light on the catalase activity of the calf eye vitreous. A total of 26 eyes were irradiated with UV-A light for 3 h and the catalase activity was compared with that of unirradiated eyes. An increase in activity by 33% was shown in the irradiated eyes. The reason for this increase is the induction of radical formation by short-wave light with subsequently intensified catalase formation or release to enable binding of the radicals formed by the cells. The vitreous must therefore be protected from UV-A light.