A human, Fc-optimized, immunoglobulin G1 (IgG1) monoclonal antibody directed against the FLT3 tyrosine kinase receptor (CD135), with potential antineoplastic activity. Upon binding to FLT3, anti-FLT3 monoclonal antibody 4G8-SDIEM blocks FLT3 ligand binding to FLT3 and subsequent phosphorylation of FLT3, which may result in the inhibition of FLT3-mediated signal transduction pathways. In addition, this agent may stimulate an anti-FLT3 antibody-dependent cell-mediated cytotoxicity (ADCC) against FLT3-expressing tumor cells, which may lead to the inhibition of cellular proliferation and decreased survival in FLT3-expressing cells. FLT3 (FLK2), a class III tyrosine kinase receptor, is overexpressed or mutated in most B lineage and acute myeloid leukemias.