Treatment of mice with the recently cloned hemopoietic growth factor Flt3 ligand (FL; 10 microg/day for 10 days) resulted in a large increase in myeloid lineage cells within the liver. While the number of nonparenchymal cells (NPC) harvested from liver increased about 9-fold, a 90-fold increase was observed in the proportion of CD11c+ dendritic cells (DC) recovered from NPC following overnight (18-h) culture in granulocyte-macrophage CSF. In contrast, only a 50% increase was seen in CD11c+ cells within heart single cell suspensions and in the number of DC obtained from hearts after 18-h culture. Liver NPC and heart cell suspensions freshly isolated from 10-day FL-treated animals exhibited increased T cell allostimulatory capacity compared with controls. Overnight cultured DC from livers of FL-treated animals expressed both higher levels of costimulatory molecules (CD80 and CD86) and allostimulatory activity than those from controls. Heart-derived DC also displayed enhanced stimulatory capacity. Pretreatment of organ donors with FL for either 5 or 10 days before transplant of organs to normal recipients abrogated the spontaneous liver allograft acceptance normally observed and resulted in delayed or acute graft rejection (median survival times, 40 and 12 days, respectively). Heart rejection was significantly accelerated by pretreatment of donors with FL for 5 or 10 days (median survival times, 8 and 7 days, respectively, vs 12 days in controls). These novel findings reveal the potent immunologic adjuvant properties of FL in vivo. They also show that substantial augmentation of the number of potential allostimulatory cells in donor organs before transplantation favors rejection rather than tolerance induction.