In order to study the effect of excessive iodine on immune function of lymphocytes and the role of selenium supplementation with excessive iodine intake, the changes of T lymphocyte number, ratio of subsets, activity of natural killer (NK) cells and lymphocytes proliferation response were investigated. 150 female BALB/C mice were randomly divided into 5 groups in terms of their body weight (n=30 in each group), and 10 of each group were taken as one batch for test. Mice in the 5 groups were orally administrated with iodine 0 (group I), 1500 (group II), 3000 (group III), 6000 microg/L (group IV), iodine 6000 microg/L plus selenium 0.3 mg/L (group V) respectively for 30 days Lymphocyte proliferation response, CD4(+)/CD8(+), Th1/Th2 and the activity of NK cells were measured. CD4(+)/CD8(+) was significantly lower, while lymphocyte proliferation response stronger, and Th1/Th2 and the activity of NK cells significantly higher in group IV than in group I (P<0.01). There was no significant difference in all indexes between group V and group I (P>0.05). It was suggested that excessive iodine as exogenous chemical materials can induce disorders of T lymphocyte immune function in mice. 0.3 mg/L selenium supplementation can protect mice against toxicity induced by 6000 microg/L iodine.