This study examined the effect of fluorescent light on the timing and severity of age-related cataracts in a fully pigmented mouse strain, the (C57BL/6×C3H)F1, that normally develops slowly progressing age-related cataracts only beyond middle age. Two groups of 56 animals each were exposed, respectively, either to a daily range of 66–222 foot candles (FC) or to 1FC of standard fluorescent lighting for a period beginning at 5 weeks of age and ending at 33.5 months (by which time approximately 65% of the colony had died). Contrary to previous reports involving albino rats or mice and a strain of pigmented but cataract-prone transgenic mice, the two groups of animals in this experiment did not differ for cataract development in time of first occurrence, rate of advancement, or degree of severity. It was concluded that genetic predisposition, based on levels of oxidative free radical production vs antioxidant enzyme and repair enzyme protection in the lens, was probably the major factor governing the rate and degree of age-related cataract development in these animals. The effect of relatively intense life-long fluorescent light exposure was so minimal as not to be manifested in this strain of mice under the conditions of this experiment. Remarkably, maintaining the one group of mice in semi-darkness from 5 weeks of age to beyond their mean lifespans did nothing to delay or reduce the incidence or severity of their age-related cataracts.