In experiments on 406 CBA mice the effect of antimicrobial preparations, used in combination with total gnotobiotic isolation, on the survival rate of totally irradiated animals was studied. The use of antimicrobial preparations was shown to considerably enhance the survival rate of these animals. Cyprofloxacin, a new preparation of the kinolone row, exhibited a good effect ensuring selective decontamination of the intestine. The use of cyprofloxacin prior to irradiation with the subsequent isolation of the animals in gnotobiotic chambers increased the survival rate of these animals by 2.7 times. But the maximum survival rate of irradiated mice was observed in those cases when antimicrobial preparations were used both before and after irradiation in combination with the total gnotobiotic isolation of the animals.