Selenium (Se) has received a lot of attention for its antioxidant and immune modulating properties. Yet, comparably few studies have focused on the horse. Therefore the objectives of this research were to evaluate the influences of Se status on immune function and antioxidant defense in horses. Twenty eight horses were allocated to one of 4 dietary Se treatments: low (LS), adequate (AS), high organic (SP) and high inorganic (SS). First, horses assigned to LS, SP and SS were depleted of Se and received a low Se diet (0.07 ppm Se) for 35 wk, while AS received an adequate Se diet (0.14 ppm Se). During week 28 to 35 immune function was evaluated using a vaccine challenge with keyhole limpet hemocyanin (KLH) and equine influenza as antigens. Then, a 29 wk repletion phase followed. The LS and AS received the same diets described above while SP received an organic Se supplemented diet (0.3 ppm; Sel-Plex, Alltech, Nicholasville, KY) and SS an inorganic Se supplemented diet (0.3 ppm; sodium selenite). Immune function was assessed using a vaccine challenge with ovalbumin (OVA) and equine influenza as antigens during week 22 to 29. Samples collected throughout the depletion and repletion phases were used to assess change in Se status, antioxidant status and oxidative stress. Finally, a mild exercise test served to assess exercise induced oxidative stress. The experimental model responded as hypothesized, evaluated by blood Se and glutathione peroxidase (GPx) activity. Upon vaccination with KLH, antibody response was faster in AS than LS. Antigen specific mRNA expression of T-bet was also higher for AS than LS. Following OVA vaccination humoral and cell-mediated vaccination responses were similar across treatments. However, non-specific stimulation of peripheral blood mononuclear cells indicated suppressed mRNA expression of selected cytokines for LS compared to AS, SP and SS. Antioxidant capacity and oxidative stress were unaffected by change in Se status. A difference in GPx response post exercise was also noted between SP and SS. Low Se status impaired some measures of immune function. Supplementation at 0.3 ppm may benefit horses as indicated by higher GPx activity in idle and exercised horses.