The possible multiple effect of galactose diet and X-irradiation on cataract development in mice was investigated by scanning electron microscopy. X-ray cataracts were induced by maintaining animals either on a normal or 50% galactose diet. No detectable changes in lens morphology were seen in animals fed galactose alone compared with normal controls. 2 months after X-ray, less damage had occurred to the lenses of the galactose-fed animals compared to those on a normal diet. At 4 months after X-ray, animals on a normal diet developed mature cataracts, while in the galactose-fed animals they were delayed by 0.5-1.5 months. Suppression of the development of X-ray cataract by galactose feeding may be related to the ability of this sugar, which accumulates in mouse lens due to the low levels of aldose reductase, to act as a free radical scavenger.