The effects of dietary l-arginine (Arg) on immunosuppression following infectious bursal disease virus (IBDV) inoculation in broiler chickens were evaluated. The design of this study was a 5 × 2 factorial arrangement (n = 5) with five Arg concentrations (starter: 9.9, 13.9, 17.6, 21.3 and 25.3 g kg(-1) ; grower-finisher: 9.5, 13.5, 17.1, 20.1 and 23.6 g kg(-1) ) with or without IBDV inoculation (IBDV or saline inoculation at 14 days). Chickens were sampled at 2, 4 and 6 days post-inoculation (DPI) and 42 days of age. The IBDV inoculation decreased (P = 0.05) CD3(+) , CD4(+) , and CD8(+) T cell counts at 2 days post-inoculation (DPI) and monocyte counts at 6 DPI; and reduced (P < 0.05) bursal interleukin-1β (IL-1β) mRNA expression at 2 DPI and serum IL-6 concentration at 4 DPI. Increasing Arg concentration increased (P < 0.05) CD4(+) and CD8(+) T cell counts at 2 DPI, linearly increased (P = 0.05) CD3(+) T cell counts in IBDV-inoculated groups and monocyte counts in control groups at 4 DPI; increased (P < 0.05) serum IL-6 concentration in IBDV-inoculated groups at 2 DPI; and increased (P < 0.05) serum anti-IBDV antibody titres at 42 days of age. Varying concentrations of Arg supplementation attenuated IBDV inoculation induced immunosuppression via modulating circulating T cell sub-populations.