Low-level irradiation (1.2-2.4 cGy, dose-rate 0.6 cGy/day) leads to a significant acceleration of the development of spontaneous leukosis in AKR mice: decrease in the average and the maximum life-span of animals leukosis-carriers (by 20 and 120 days, respectively) and an increase in the leukosis incidence rate (%). The introduction of antioxidant phenozan (beta-(4-hydroxy-3,5-ditertbutylphenyl) propionic acid) results in a considerable antileukosis effect: the average life-span increases by more than 40 days and the leukosis incidence rate decreases by 6%.