[en] The author has studied radiation-induced chemical reaction processes at the cell level, with special emphasis of free radical processes leading to the final degradation products. This paper focuses on radical intermediates at the sugar moiety, as well as at the base moiety, in nucleic acids, which have been identified by the spin-trapping/ESR technique, and on their chemical structures. Studies on halogenated pyrimidines, such as BrUdR and 2'-Cl-TdR, involved in radiation sensitization are also referred to. (Namekawa, K.)