Abstract: In order to study the effect of tryptophan (Trp) on antioxidant capacity and immunity in stressed mice, 80 mice were randomly divided into four groups:the normal control group (NC) , cytoxan group (CTX) , cytoxan-low dose tryptophan group (CTX-L) , and cytoxan-high dose tryptophan group (CTX-H) .During the experiment, intraperitoneal injection of cytoxan (100mg/kg·bw) for 3d, then intragastric administration of saline and tryptophan (100, 200mg/kg·bw) for 2 weeks.Results:Compared with NC group, the level of plasma malondialdehyde (MDA) had no change, tissue MDA level increased significantly.The activity of superoxide dismutase (SOD) in plasma and tissues remarkably decreased, glutathione peroxidase (GSH-Px) content and thymus index lowered in CTX group, however, Spleen Index increased.The levels of interferon-γ (IFN-γ) , and interleukin-6 (IL-6) decreased to different extent.5-hydroxytryptamine (5-HT) of hypothalamus decreased significantly.Compared with CTX group, in CTX-L group, the amount of MDA in tissues descended significantly.And the activity of plasma SOD recovered partly in both CTX-L and CTX-H groups, however, both groups had no effect on tissue SOD.Plasma GSH-Px increased significantly in both CTX-L and CTX-H groups, spleen GSH-Px had no significantly, however, cardiac muscle decreased significantly.The content of IFN-γ, IL-6 all enhanced significantly in CTX-H group.The content of 5-HT in hypothalamus in stressed mice was promoted significantly both in CTX-L and CTX-H groups.Conclusion:The results indicated that supplement of tryptophan involves increase of antioxidant capacity of stressed mice, and enhancement of the body's immunity.