Strontium 90 (⁹⁰Sr) remains in the environment long after a major nuclear disaster occurs. As a result, populations living on contaminated land are potentially exposed to daily ingesting of low quantities of ⁹⁰Sr. The potential long-term health effects of such chronic contamination are unknown. In this study, we used a mouse model to evaluate the effects of ⁹⁰Sr ingestion on the immune system, the animals were chronically exposed to ⁹⁰Sr in drinking water at a concentration of 20 kBq/l, for a daily ingestion of 80–100 Bq/day. This resulted in a reduced number of CD19 B lymphocytes in the bone marrow and spleen in steady-state conditions. In contrast, the results from a vaccine experiment performed as a functional test of the immune system showed that in response to T-dependent antigens, there was a reduction in IgG specific to tetanus toxin (TT), a balanced Th1/Th2 response inducer antigen, but not to keyhole limpet hemocyanin (KLH), a strong Th2 response inducer antigen. This was accompanied by a reduction in Th1 cells in the spleen, consistent with the observed reduction in specific IgG concentration. The precise mechanisms by which ⁹⁰Sr acts on the immune system remain to be elucidated. However, our results suggest that ⁹⁰Sr ingestion may be responsible for some of the reported effects of internal contamination on the immune system in civilian populations exposed to the Chernobyl fallout.