Aims To determine the effects of Lactobacillus fermentum I5007 on the redox state of piglets oxidatively stressed with diquat. Methods and Results Twenty-four, 28-day-old barrows were used in a 2x2 factorial design experiment with the main effects being Lact.fermentum supplementation and diquat challenge. Half of the pigs (n=12) were orally administered with 20ml of a solution containing 108CFUml1 of Lact.fermentum each morning of the 21-day trial, while the remainder received saline. On day 8, these two groups were further subdivided so that half of the pigs in each group (n=6) were intraperitoneally injected with 10mgkg1 BW diquat, while the remainder received saline. The diquat-injected pigs had significantly poorer performance and increased levels of plasma cortisol, adrenaline, carbonyl and malondialdehyde. Lactobacillus fermentum supplementation significantly increased superoxide dismutase and glutathione and increased the ability to inhibit superoxide anion production in liver and muscle. Conclusions Lactobacillus fermentum improved the anti-oxidative defence system and alleviated damage caused by diquat. Significance and Impact of the Study Lactobacillus fermentum has the potential to alleviate oxidative stress and improve weaning pig performance.