Apoptosis of thymocytes is involved in the negative selection of thymus, but it remains unclear how the cell death of thymocytes is regulated by the thymic stromal cells. By using immunohistochemistry, TdT mediated dUTP nick end labeling (TUNEL) and flow cytometry methods, it was found that Fas ligand was expressed in the thymic medulla and a mouse thymic medullary type dendritic cell line (MTSC4). The DNA fragmentation and TUNEL positive staining of thymocytes were detected in 6 h of cocultures with MTSC4. 97% of the thymocytes which bound to MTSC4 were Fas antigen positive cells. The DNA fragmentation of thymocytes induced by MTSC4 was inhibited by the addition of 20 mmol/L N acetyl galactosamin monosaccharide and the pretreatment of a monoclonal antibody (PF 18 3) which recognized a putative antigen on MTSC4. These results suggested that the mechanisms of induction of cell death by stromal cells may include the interactions of multiple cell surface molecules, in addition to the Fas/Fas ligand system. Apoptosis of thymocytes is involved in the negative selection of thymus, but it remains remainsarre how the cell death of thymocytes is regulated by the thymic stromal cells. By usinghistochemistry, TdT mediated dUTP nick end labeling (TUNEL) and flow cytometry methods, it was found that Fas ligand was expressed in the thymic medulla and a mouse thymic medullary type dendritic cell line (MTSC4). The DNA fragmentation and TUNEL positive staining of thymocytes were detected in 6 h of cocultures with MTSC4. 97% of the thymocytes which bound to MTSC4 were Fas antigen positive cells. The DNA fragmentation of thymocytes induced by MTSC4 was inhibited by the addition of 20 mmol / LN acetyl galactosamin monosaccharide and the pretreatment of a monoclonal antibody (PF 18 3) which recognized a putative antigen on MTSC4. These results suggested that the mechanisms of induction of cell death by stromal cells may include the interactions of multiple cell surface molecules, in addition to th e Fas / Fas ligand system.