Mice homozygous for lpr (lymphoproliferation) or gld (generalized lymphoproliferative disease) develop lymphadenopathy and splenomegaly and suffer from autoimmune disease. The lpr mice have a defect in a cell‐surface receptor, Fas, that mediates apoptosis, while gld mice have a mutation in the Fas ligand (FasL). Northern hybridization with the FasL cDNA as probe indicated that the cells accumulating in lpr and gld mice abundantly express the FasL mRNA without stimulation. By means of in situ hybridization and immunohistochemistry, we identified the cells expressing the FasL mRNA as CD4‐CD8‐ double negative T cells. The T cells from lpr mice were specifically cytotoxic against Fas‐expressing cells. Since FasL is normally expressed in activated mature T cells these results indicate that the double negative T cells accumulating in lpr and gld mice are activated once, and support the notion that the Fas/FasL system is involved in activation‐induced suicide of T cells. Furthermore, the graft‐versus host disease caused by transfer of lpr bone marrow to wild‐type mice can be explained by the constitutive expression of the FasL in lpr‐derived T cells.