The effect of 2-mercaptopropionylglycine on the development of cataract induced by a single dose of X-ray (10 Gy) was investigated in rats. Intraperitoneal injection of 20 mg/kg, three times weekly starting 1 day after irradiation delayed the development of X-ray-induced cataracts significantly. The amounts of non-protein SH groups, malondialdehyde and the Na+/K+ratio, in the lenses of rats post-treated with the drug were significantly maintained at normal levels even at 27 weeks after irradiation.On the other hand, a single administration of 250 mg/kg of the compound, 30 min prior to irradiation had no effect on cataract progression induced by X-ray.