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The aim of this work was to determine the possible occurrence of damage to the immune system during the course of chronic ingestion of 137Cs. BALB/C mice were used, with 137Cs intake via drinking water at a concentration of 20kBql-1. Adults received 137Cs before mating and offspring were sacrificed at various ages between birth and 20 weeks. Phenotypic analysis of circulating blood cells and thymocytes did not show any significant modification of immune cell populations in animals ingesting 137Cs as compared with control animals, with the exception of a slight increase in Treg percentage at the age of 12weeks. Functional tests, including proliferative response to mitogens such as phytohaemagglutinin, response to alloantigens in mixed lymphocyte reaction and immunoglobulin response to vaccine antigens such as tetanus toxin and keyhole limpet haemocyanin did not show any significant functional modification of the immune system in 137Cs-ingesting animals as compared with control animals. Overall, our results suggest that chronic ingestion of a low concentration of 137Cs in drinking water in the long term does not have any biologically relevant effect on the immune system. © 2011 IOP Publishing Ltd.