Mesna has been used with ifosfamide to prevent urotoxicity in the treatment of testicular cancers. This drug also protected the toxicities of adriamycin without compromising cytostatic activity. With n idea of radioprotective role of sulfhydryl group of radioprotectors and of mesna decreasing the toxic effect of adriamycin which produces free radicals, mesna and radiation were administered to mice to study the protective effect of this drug and to identify the difference in regenerative capacity of the germ cells in the testis between radiation-treated and both mesna- and radiation-treated groups. The shape and numbers of spermatogenic cells in the seminiferous tubules were examined every week after irradiation. In both groups, initial reduction and later recovery in germ seel numbers and shape was observed. The lowest germ cell number was found around three weeks after irradiation. Mean germ cell number of the mesna-treated group was significantly higher than radiation-treated group at all observed periods (p<0.05). More competent regeneration was present in mesna-treated group. These results suggest that mesna protect the testis from radiation injury. Further study will be necessary to identify whether mesna protects other tissues from radiation and it does not hamper tumor control.