The immunological status of CBA mice surviving 2-11 months after X-irradiation in a dose 5 Gy was investigated. The immune response against sheep erythrocytes in the spleen recovered in 5 months after irradiation, but in the lymph nodes it was not normalized in 8 months. The hypoplasia of thymus and lymph nodes was observed in all irradiated mice whereas the cellularity of the spleen and bone marrow and the T/B-lymphocyte ratio in all lymphoid organs did not differ from normal. The dependence of the immune response upon the antigen dose was less pronounced in the irradiated animals than in the controls. The dynamics of the response in compared groups was similar. The results obtained indicate that there are distinctions between late radiation-induced immunodepression and immunologic insufficiency during acute radiation sickness and that there is organ specificity in the postirradiation recovery of different parts of the lymphoid system.