T o understand the relationship between host antigen-presenting cells (APCs) and donor T cells in initiating graft-versus-host disease (GVHD), we followed the fate of host dendritic cells (DCs) in irradiated C57BL/6 (B6) recipient mice and the interaction of these cells with minor histocompatibility antigen- (miHA-) mismatched CD8⁺ T cells from C3H.SW donors. Host CD11c⁺ DCs were rapidly activated and aggregated in the T cell areas of the spleen within 6 hours of lethal irradiation. By 5 days after irradiation, <1% of host DCs were detectable, but the activated donor CD8⁺ T cells had already undergone as many as seven divisions. Thus, proliferation of donor CD8⁺ T cells preceded the disappearance of host DCs. When C3H.SW donor CD8⁺ T cells were primed in vivo in irradiated B6 mice or ex vivo by host CD11c⁺ DCs for 24–36 hours, they were able to proliferate and differentiate into IFN-γ–producing cells in β₂-microglobulin–deficient (β₂m^–/–) B6 recipients and to mediate acute GVHD in β₂m^–/– → B6 chimeric mice. These results indicate that, although host DCs disappear rapidly after allogeneic bone marrow transplantation, they prime donor T cells before their disappearance and play a critical role in triggering donor CD8⁺ T cell–mediated GVHD.