In this paper, the change of lymphocytes micronuclear frequency in rabbits IN VIVO and IN VITRO irradiated by 100, 200, 300 and 400 rads is reported. The results indicate that micronuclear frequency was increased with the dose of radiation in both conditions mentioned above, but there was no marked difference. Again, these results show that the lymphocytes micronuclear frequency used as a biodosimeter was completely possible.,The lymphocytes micronulear frequency increased immediately 2 hours later after radiation and the frequency increase maintained for 2 weeks, starting from 4 th week the micronuclear frequency was reduced. All these results were discussed.