DURING mammalian development, many cells are programmed to die^1,2 most mediated by apoptosis³. The Fas antigen⁴ coded by the structural gene for mouse lymphoproliferation mutation (lpr)^5,6, is a cell surface protein belonging to the tumour necrosis factor/nerve growth factor receptor family^7,8, and mediates apoptosis⁷. The Fas antigen messenger RNA is expressed in the thymus, liver, heart, lung and ovary⁸. We prepared a monoclonal antibody against mouse Fas antigen, which immunoprecipitated the antigen (M_r 45K) and had cytolytic activity against cell lines expressing mouse Fas antigen. We report here that staining of mouse thymocytes with the antibody indicated that thymocytes from the wild-type and lpr^cg mice expressed the Fas antigen, whereas little expression of the Fas antigen was found in lpr mice. Intraperitoneal administration of the anti-Fas antibody into mice rapidly killed the wild-type mice but neither lpr nor lpr^cg mice. Biochemical, histological and electron microscope analyses indicated severe damage of the liver by apoptosis. These findings suggest that the Fas antigen is important in programmed cell death in the liver, and may be involved in fulminant hepatitis in some cases.