An immunoconjugate consisting of the humanized monoclonal antibody huMy9-6 conjugated to the cytotoxic maytansinoid DM4 with potential antineoplastic activity. The monoclonal antibody portion of anti-CD33 monoclonal antibody-DM4 conjugate AVE9633 specifically binds to the cell surface antigen CD33 expressed on myeloid leukemia cells; upon internalization, the DM4 moiety is released, binds tubulin, and disrupts microtubule assembly/disassembly dynamics, resulting in the inhibition of cell division and cell growth in myeloid leukemia cells that express CD33. CD33 is expressed on normal non-pluripotent hematopoietic stem cells as well as on myeloid leukemia cells.