The effect of a 30% galactose diet on the progression of X-ray-induced cataract in mice was evaluated by following morphological changes as seen by light and transmission electron microscopy in different regions of the lens. Lens opacities as observed with the slit-lamp biomicroscope developed at a slower rate in galactose-fed animals than in those on a normal diet. The protective effect of galactose on X-ray cataract was seen whether galactose feeding was initiated either 1 week before or after exposure to X-ray. At 4 months after X-ray approximately 50% of galactose-fed animals had mature cataracts, compared to 100% in the control group. Similarly, at two weeks after exposure to X-ray, before any lens opacities were observed, morphological changes were more severe in the control group; cells in the meridional row were more disorganized in the control than in the galactose-fed groups. However, the progression of mature cataracts in the two galactose-fed groups were not significantly different. Since free radicals produced by X-ray are thought to be short-lived, the protective effect of galactose feeding after X-ray was unexpected, raising the possibility that some of the active species may be long-lasting. The nature of such radicals, if any, is unknown and remains to be investigated.