The reaction rate of metallothionein (MT) with hydroxyl radical was determined to be ca. 10,12,mol,-1,•L•s,-1, by studying the kinetic competition of MT with ferrocyanide for OH which was generated in situ by irradiation. Radioprotective effect of MT on thymine was studied in N,2,O-saturated phosphate buffer solutions at pH 7.0. Both isoforms of rabbit MT can protect thymine against radiation damage effectively, and their effectiveness is in the order of ZnMT-1 ,>, ZnMT-2,>,(Cd,Zn) MT-1 ,>,(Cd,Zn)MT-2. However,the dose reducing factor was less than expected, which was possibly due to the secondary reactions of RS with thymine or T(OH) radicals with MT. Radiation effect on thymine in O,2,-saturated 0.1 mol•L,-1, HCOONa Solution was also studied, and the results showed that O,2,-, and H,2,O,2, didn't cause direct damage to thymine.