As an integral constituent of glutathione peroxidase (GSH-Px), Se is a vital antioxidant during stress. Calves are exposed to multiple factors that can lead to oxidative stress, including viral infection. Our objective was to evaluate the effects of supplementation of Se and Se source on immune function of Holstein steer calves challenged with infectious bovine rhinotracheitis virus (IBRV). Twentyfour Holstein steers (initial BW = 170 ± 0.6 kg) were assigned randomly to 3 treatments: 1) no supplemental Se (control); 2) 1 mg/steer daily of Se from Se yeast (Sel-Plex, Alltech Inc., Nicholasville, KY); or 3) 5 mg/steer daily of Se from sodium selenite. Treatments were fed for 35 d before intranasal inoculation with IBRV (d 0) and were continued through 21 d after the challenge. Whole blood Se was greater (P > 0.001) in steers receiving Sel-Plex than in the other 2 groups, but GSH-Px activity (P < 0.80) was not affected by treatment. Serum interleukin-1ß concentrations tended to be greater (P = 0.09) in steers fed selenite compared with control steers, and tumor necrosis factor-a concentrations 12 h after the challenge tended (P = 0.09) to be greater in steers fed Sel-Plex compared with control steers. Steers in the selenite group had decreased (P = 0.02) serum urea N at 4 and 72 h after the challenge compared with control steers, and steers receiving Sel-Plex had increased serum urea N at 72 h (P = 0.008) after the IBRV challenge compared with control steers. Serum glucose concentrations tended (P = 0.06) to be less in steers that received the selenite treatment versus no supplemental Se. Serum concentrations of nonesterified fatty acids (P > 0.40), insulin, cortisol, prolactin, and triiodothyronine were not affected (P = 0.24) by treatment. The present data help to define the immune response to a viral challenge in cattle. Additional research is needed to clarify the role of Sel-Plex Se in the immune response of cattle to viral infections.