Both tumor necrosis factor α (TNFα) and Fas ligand (FasL) have been implicated in the pathogenesis of graft-versus-host disease (GVHD). In this study, we examined the ameliorating effects of neutralizing anti-FasL and/or anti-TNFα monoclonal antibody (MoAb) in a lethal acute GVHD model in mice. Whereas the treatment with either anti-FasL or anti-TNFα MoAb alone significantly delayed the mortality and improved the body weight, a complete protection was achieved by the administration of both MoAbs. Pathological examination indicated differential effects of anti-FasL or anti-TNFα MoAb on GVHD-associated pathologies. Hepatic lesion was improved by anti-FasL but not anti-TNFα MoAb. In contrast, intestinal lesion was improved by anti-TNFα but not anti-FasL MoAb. Cutaneous and splenic lesions were improved by either MoAb. The combination of both MoAbs improved all these lesions. These results indicate that FasL and TNFα differentially contribute to the GVHD pathologies and a complete protection from mortality can be achieved by neutralization of both FasL and TNFα.