One-day-old chickens were divided into two groups and reared under similar conditions. One group was fed a diet supplemented with 1000ppm ascorbic acid and the other group was fed an identical diet, but not supplemented with ascorbic acid. Both groups were vaccinated against infectious bursal disease (IBD) at 7 days of age and challenged orally with 4x10(5) of 50% embryo-lethal-dose IBDV 14 days later. The number of anti-IBDV antibody secreting cells, production of interleukin-2 (IL-2) by splenocytes, number of CD4(+), CD8(+) and IgM(+) cells in spleen and IgM(+) cells in bursa of Fabricius were compared between the two groups at 7 days (prior to vaccination), 21 days (14 days post-vaccination and prior to challenge) and 31 days (10 days post-challenge) of age. The number of CD8(+) in spleen at 7 days of age and IgM(+) cells in bursa at 7, 21 and 31 days of age were significantly higher in ascorbic acid supplemented group (P<0.05). Production of IL-2 by splenocytes was higher as indicated by higher stimulation indices in ascorbic acid supplemented group. The number of anti-IBDV IgG antibody secreting cells in spleen at 21 and 31 days of age were significantly higher in ascorbic acid supplemented group (P<0.05). Dietary supplementation of ascorbic acid may ameliorate the immunosuppression caused by IBDV vaccination and improve humoral and cellular immune responses.