Dendritic cells isolated from lymphoid tissues are potent stimulators of primary allogeneic T-cell responses in vitro and in vivo. Similar major histocompatibility complex class II-bearing dendritic-shaped leukocytes are contained within transplanted organs and these are thought to be important passenger leukocytes that trigger rejection. Recent findings on the migration, phenotype, and function of cardiac dendritic leukocytes (DLs) are reviewed. After transplantation donor DLs migrate rapidly from mouse cardiac allografts into the recipients's spleens. Within the spleens donor DLs associate with recipient CD4+ T cells. Isolated cardiac DLs, like lymphoid dendritic cells, are potent stimulators of T-cell proliferation in vitro. This suggests that DLs function as passenger leukocytes by migrating from grafts into the lymphoid tissues of the recipient and that sensitization to vascularized organ allografts may occur centrally within lymphoid tissues rather than peripherally in the graft itself.