Since the preliminary proposal of "free radical repair" via H-transfer process was suggested, a series of ESR studies, such as by Ormerod and Alexander, Pullman and Milvy et al.,[1], have been performed to clarify the repair mechanism exerted by the sulfhydryl compounds such as octyl mercaptan, cysteine, but only cysteamine was studied in detail in binary system containing DNA or dTMP and cysteamine. As for cysteine it was failed to be confirmed in binary system containing cysteine •HCl and DNA or dTMP.,In our previous work cysteine paramethyl benzenesulfonate was used to instead of its hydrochloride, thus ESR spectrum from dTMP-cysteine paramethyl benzenesulfonate irradiated and recorded at 77K revealed predominately characteristic from dTMP, but after a long range vaiable temperature from 189 to 323K accompanying the decay of ESR spectrum from dTMP the signal from cysteine grew gradually and a stable state assigned to secondary organosulfur radical RCH,2,SS• or RCHS•-SH,2,CR) attained at higher temperature (323K) or long time (~20 hrs) annealing at room temperature.,Recently, the ESR spectrum from DNA-cysteine paramethyl benzenesulfonate irradiated and recorded at 77K revealed the charcteristic similar to that from cysteine alone. After annealing at room temperature ESR pattern also changed into that from cysteine alone, no trace signal from 5-thymyl radical can be found.