PF18-3 monoclonal antibody (mAb), one of the rat mAbs against mouse thymic stromal cells (MTSC), has been found to inhibit thymocyte apoptosis induced by a mouse thymic dendritic cell line, MTSC4, in previous co-culture study. The aim of this research is to investigate the character of PF18-3 mAb recognized molecule ( PF18-3 molecule) and its role in MTSC4-induced thymocyte apoptosis. The characterization of PF18-3 molecule expression has been conducted by FACS analysis. PF18-3 molecules have been found to express on MTSC4 as well as on Con A activated but not freshly isolated thymocytes. Up-regulated expression of PF18-3 molecules has been also observed on thymocytes after being co-cultured with MTSC4 for 48 h. The results from FACS analyses by Pl staining for detecting apoptosis-related hypodiploid and by PF18-3 mAb staining reveal that PF18-3 molecules expresss specifically on the apoptotic subgroup of thymocytes with high hypodiploid content. The PF18-3 molecule expressed on apoptotic thymocytes wit PF18-3 monoclonal antibody (mAb), one of the rat mAbs against mouse thymic stromal cells (MTSC), has been found to inhibit thymocyte apoptosis by a mouse thymic dendritic cell line, MTSC4, in previous co-culture study. The aim of this research is to investigate the character of PF18-3 mAb recognized molecule (PF18-3 molecule) and its role in MTSC4-induced thymocyte apoptosis. The characterization of PF18-3 molecule expression has been conducted by FACS analysis. PF18-3 molecules Up-regulated expression of PF18-3 molecules has been also observed on thymocytes after being co-cultured with MTSC4 for 48 h. The results from FACS analyzed by Pl staining for detecting apoptosis-related hypodiploid and by PF18-3 mAb staining reveal that PF18-3 molecules expresss specifically on the apoptotic subgroup of thymocytes with high hypodiploid content. The PF18-3 molecule expressed on apopto tic thymocytes wit