Avian infectious bronchitis virus (IBV) causes high mortality and reduced production even in vaccinated birds. The study hypothesis was a decoction of Chinese herbal medicine (modified Ma Xing Shi Gan Tang), in the drinking water would reduce mortality, improve recovery and antioxidant capabilities in chickens infected with IBV. Two hundred 15-day-old chickens were randomly divided into 5 groups of 40 each. One group was an untreated control group. Chickens in the other 4 groups were inoculated intranasally with IBV and all developed reduced activity, appetite and respiratory signs within 48 hours. Chickens in 3 groups received 0.6%, 0.4% or 0.2% modified Ma Xing Shi Gan Tang twice daily in the drinking water for 5 days beginning 48 hours after IBV inoculation. Clinical signs were monitored. Deaths and recoveries recorded. Serum superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) and malondialdehyde (MDA) were evaluated on 6 randomly selected chickens from each group, 5 times over a 3-15 day period and the M±SD statistically compared. Mortality was very significantly (p<0.01) reduced from 42% in the untreated group to 5%, 10% and 27% (0.6%, 0.4% or 0.2% herbs respectively) and affected birds recovered with no adverse effects from the herbs. Chickens receiving 0.6% or 0.4% modified Ma Xing Shi Gan Tang had significantly (p<0.01) higher serum SOD and GSH-Px and lower serum MDA than untreated IBV chickens. The antioxidant properties of modified Ma Xing Shi Gan Tang make it a novel non-toxic medication for the treatment and potential prevention of IBV infections of chickens.