Effect of 2-Marcaptopropionyl Glycine (MPG) on radiation-induced mitotic delay in cultured manmalian cell (L-5) was studied. The mitotic delay induced by irradiation of 200 rads was protected with treatment of MPG (20-100mM). A recovery curves of mitotic index became steep with increasing of MPG concentration. But the mitotic delay time was not shortened by MPG. On the basis of these data, the mechanisms of the radioprotective action of MPG were discussed.