The class III receptor tyrosine kinase, FLT3, is expressed by >90% of B-lineage acute lymphoblastic leukemias (ALL) blasts. In addition, it is expressed at extremely high levels in ALL patients with MLL-rearrangements or hyperdiploidy and sometimes mutated in these same patients. In this report, we investigated the effects of EB10, an anti-human FLT3 monoclonal antibody capable of preventing binding of FLT3 ligand (FL), on ALL cell lines and primary cells. In vitro studies, examining the ability of EB10 to inhibit FLT3 activation and downstream signaling in ALL cell lines and primary blasts, yielded variable results. In some cell lines FLT3 phosphorylation was inhibited and with it, downstream activation of pathways involving MAPK, AKT, and STAT5 phosphorylation. However, several cell lines actually exhibited FLT3 activation upon antibody treatment, possibly because of antibody-mediated receptor dimerization, and subsequent activation of downstream pathways. Nevertheless, through antibody-mediated cellular cytotoxicity (ADCC) such an antibody could still prove efficacious against leukemia cells in vivo. In fact, EB10 treatment significantly prolongs survival and/or reduces engraftment of ALL cell lines and primary ALL blasts in NOD/SCID mice. This effect might be even more pronounced in a host that was less immune compromised than are NOD/SCID mice. The leukemic cells surviving EB10 treatment in the mice were characterized by FACS analysis and found to express low levels or no FLT3. In contrast to the reduction in engraftment of human ALL primary blasts, EB10 treatment of NOD/SCID mice did not reduce engraftment of human hematopoietic CD34+ cells. Taken together, these data demonstrate that EB10 is selectively cytotoxic to ALL blasts while having little effect on normal hematopoiesis. Such an antibody, either naked or conjugated to radioactive isotopes or cytotoxic agents, may prove useful in the therapy of infant ALL as well as childhood and adult ALL patients whose blasts typically express FLT3.