Allylmethylsulfide (AMS), a volatile organosulfur derivative from garlic, has been shown to have radioprotective effects in radiation-challenged cell and animal models, but the mechanism of radioprotection is not well understood. To determine the mechanism of radioprotection in an in vivo model, we first verified the antioxidant capacity of AMS using 2,2'-azobis(2-amidinopropane) dihydrochloride-induced human embryonic kidney 293T cells by measuring reactive oxygen species generation, reduced glutathione, protein tyrosine kinase/protein tyrosine phosphatase balance, and nuclear factor-kappaB (NF-kappaB) protein levels. We then investigated the protective effects of AMS (55 and 275 micromol/kg, intraperitoneal treatment) on 15 Gy X-ray-irradiated mouse kidney. The results showed that AMS decreased the free radical-induced lipid peroxidation in mice exposed to X-rays. Moreover, the antioxidative AMS suppressed the activation of NF-kappaB and its dependent genes such as vascular cell adhesion molecule-1, inducible nitric oxide synthase, and cyclooxygenase-2 through inhibition of IkappaBalpha phosphorylation and activation of IkappaB kinase alpha/beta and mitogen-activated protein kinases (MAPKs). Based on these results, AMS may be a useful radioprotective agent by down-regulating the MAPKs and NF-kappaB signaling pathway that can be induced via X-ray irradiation.