The objectives of this study were to investigate the effects of dietary high molybdenum (Mo) on immune function by determining changes of the subsets of peripheral blood T-cells and serum interleukin (IL)-2 contents. 300 1-day-old avian broilers were divided into four groups and fed on a corn-soybean basal diet as control diet or the same diet amended to contain 500; 1,000; and 1,500 mg/kg of Mo supplied as sodium molybdate dihydrate. In comparison with those of the control group, the percentages of CD3(+), CD3(+)CD4(+) and CD3(+)CD8(+) were decreased in 1,000 and 1,500 mg/kg of Mo intake groups from 14 days of age to 42 days of age. Also, the serum IL-2 contents were decreased in 1,000 and 1,500 mg/kg of Mo intake groups from 14 days of age to 42 days of age. Histopathologically, hypocellularity appeared in the thymus in 1,000 and 1,500 mg/kg of Mo intake groups. It was concluded that dietary high-Mo (1,000 mg/kg and 1,500 mg/kg) reduced the percentages of peripheral blood T-cell subsets and serum IL-2 contents and caused thymic lesions. The cellular immune function was finally injured in broilers.