The expression of Fas ligand on natural killer (NK) cells and Fas-mediated cytotoxicity by NK cells was investigated. Fas ligand mRNA was expressed in freshly isolated NK cells but not in T cells. Furthermore, the Fas ligand was detected on the cell surface of NK cells by staining with soluble Fas molecule. We analyzed the cytolytic activity of NK cells against thymocyte targets from normal and lpr mice, and found that the NK cells killed thymocytes from normal mice but not from lpr mice. On the other hand, splenic T cells did not show any cytotoxicity against either of the thymocyte targets. Similarly, NK cells exhibited cytotoxicity against transfectants expressing Fas antigen but not against parental cells or transfectants expressing a mutant Fas antigen with deleted cytoplasmic region. These results demonstrated that NK cells express Fas ligand and possess the capability of killing target cells expressing Fas antigen on their surface. This finding suggests that NK cells play an important role by eliminating Fas-expressing cells either constitutively or inducibly in peripheral lymphoid organs.