D ENDRITIC cells (DC) that express high levels of MHC class II and costimu!atory molecules are the most potent antigen-presenting cells and the only population that can activate naive T cells. 1 Evidence has also accumulated, however, that DC can playa role in tolerance induction. Thus, thymic DC induce central tolerance to MHC antigens.: The ubiquitous presence of donor DC in long-surviving human and animal organ allograft recipients suggests that DC may hold the key to the induction of transplantation tolerance.3.4 We have reported that costimulatory molecule-deficient DC induce alloantigen-specific unresponsiveness in vitro,S and prolong allograft survival.°.7 The mechanisms by which tolerance can be induced by DC, however, remain unclear. A recent report indicates that CD8+ Fas-L + (CD95L +) mouse lymphoid tissue DC can induce apoptosis in activated T cells in vitro.s The present study was undertaken to evaluate the capacity of in vitro propagated DC to induce allogeneic T-cell apoptosis and to determine the possible regulatory roles of CD95 (Fas/Apo-I) ligand and B7 molecules expressed on DC.