Although the essential role of selenium for cellular immune responses is obvious, delineation of the functions is lacking because selenium can either promote or inhibit cell growth, cytokine production, and activation of transcription factor nuclear factor-κB (NF-κB). Studies with human thioredoxin-1 (Trx-1)-transgenic (Tg) mice were conducted to evaluate the relationship between stimulation of T-cell mitogenic response by sodium selenite and the intracellular Trx-1 levels, and the activities of selenoenzymes and NF-κB–DNA binding. Concanavalin A-induced mitogenesis of wild-type mouse splenic cells was stimulated by exposure to low levels of selenite (0.02–0.1 µM), with augmentation of NF-κB–DNA binding activity. Treatment with NF-κB nuclear translocation inhibitor SN50 or thioredoxin reductase (TR) inhibitor aurothioglucose depressed this stimulatory action. The mitogenic response of Trx-1-Tg mouse splenic cells was enhanced by exposure to relatively high levels of selenite ($ 0.05 µM), compared with the wild-type mouse. Selenite also augmented TR activity but not cellular glutathione peroxidase activity in the Trx-1-overexpressed cells. These results suggest that the stimulation of T-cell mitogenic response by the physiological levels of selenite is predominantly caused by increased TR activity, which may lead to reduction of Trx-1 dependent on the intracellular expression level and promotion of DNA binding of NF-κB.