The Fas antigen (Fas) belongs to the tumor necrosis factor (TNF)/nerve growth factor receptor family, and it mediates apoptosis. Using a soluble form of mouse Fas, prepared by fusion with human immunoglobulin Fc, Fas ligand was detected on the cell surface of a cytotoxic T cell hybridoma, PC60-d10S. A cell population that highly expresses Fas ligand was sorted using a fluorescence-activated cell sorter, and its cDNA was isolated from the sorted cells by expression cloning. The amino acid sequence indicated that Fas ligand is a type II transmembrane protein that belongs to the TNF family. The recombinant Fas ligand expressed in COS cells induced apoptosis in Fas-expresslng target cells. Northern hybridization revealed that Fas ligand is expressed in activated splenocytes and thymocytes, consistent with its involvement in T cell-mediated cytotoxicity and in several nonlymphoid tissues, such as testis.