Several anti-Fas ligand (FasL) inhibitory mAb (FLIM) were raised and characterized in this study. One, FLIM58, showed more potent neutralizing activity than Fas-Fc, the previously established artificial neutralizing agent for FasL. Several murine models of acute graft-versus-host disease (GVHD) after allogeneic bone marrow transplantation have been used to show that both FasL and perforin, the major effector molecules of cytotoxic T lymphocytes, are involved in this disease. In our GVHD model, FasL rather than perforin was associated with lethality. Administration of FLIM58 or Fas-Fc reduced the weight loss and mortality caused by GVHD, although other signs of GVHD, such as skin lesions, lymphoid hypoplasia and mononuclear cell infiltration in the liver, did not improve significantly. FLIM58 was more effective than Fas-Fc in reducing mortality. Our results demonstrated that neutralizing agents for FasL are therapeutic for lethal GVHD.