Epidemiological studies in some areas of high natural radioactivity did not demonstrate a rise in cancer mortality in accordance with the increased exposure of the population to low level radiation as suggested by the linear model. Studies on inhabitants in some of these areas showed increased reactivity of T lymphocytes to mitogens and enhanced unscheduled DNA synthesis （UDS）. Animal experiments also disclosed potentiation of immune functions

stimulated UDS and induction of a cytogenetic adaptive response in lymphocytes after single or chronic exposure to low dose radiation. The mechanism of immunoenhancement after low dose radiation was analyzed in terms of the cellular events in the immune organs

the systemic regulation of these processes and the molecular basis of the above changes. The possible role of proteins induced by low dose radiation in the mechanism of the cytogenetic adaptive response was discussed.