AIM Study of the immunity and nonspecific defense factors in subjects living at a territory contaminated with radionuclides at a density of 1-5 Ci/km2 after the Chernobyl accident. MATERIALS AND METHODS A total of 144 subjects aged 18 to 82 years living in the Sasovo region of the Ryazan district were examined. Three groups were distinguished with different density of contamination: 1) n = 54, 1-5 Ci/km2; 2) n = 36, conditionally pure territory; and 3) n = 54, living at the interface of the two territories. Blood analysis was carried out, nonspecific defense factors studied in the NBT test, and cellular and humoral immunity parameters investigated. RESULTS Values of the NBT test, levels of the natural inhibitory factor and IgA, counts and functional activities of T lymphocytes and their subpopulations differed but negligibly from those in subjects living at pure territories. On the other hand, the counts of large granular lymphocytes were decreased and the incidence of autoimmune reactions to thyroid hormone antigens increased in the population exposed to low-dose ionizing radiation, which might be due to incorporation of radioactive iodine. CONCLUSION The detected changes in the population exposed to low-dose radiation indicate that the history of exposure cannot be neglected, for such an exposure causes development of some diseases or alters their course.