The radiosensitivity of primary spermatocytes in pachytene stage was estimated by counting the number of spermatids in the testes of control and MPG-treated mouse after exposure to 500, 1000 and 1500 R of Co60 gamma-rays. For this purpose, control and MPG-treated mice were killed 5 days after irradiation and countings of spermatids was made in stages I and II of the tubules. It has been observed that, although there was a death of primary spermatocytes in irradiated MPG-protected groups, quantitatively significant protection was afforded by this drug at all the 3 dose-levels studied.