Monoclonal antibodies (mAbs) have become an important modality for cancer therapy. A genetically engineered, humanized anti-CD33 antibody HuM195 has demonstrated activity against over relapsed acute myelogenous leukemia (AML) and against minimal residual disease in acute promyelocytic leukemia (APL). Radioimmunotherapy with beta (beta) particle-emitting isotopes has produced significant responses while minimizing radiation exposure to normal tissues in both nonmyeloablative and myeloablative regimens. Targeted alpha (alpha) particle therapy with 213Bi-labeled HuM195 offers the possibility of more selective tumor cell kill. Additionally, directed chemotherapy using an anti-CD33-calicheamicin conjugate (CMA-676) has produced remissions in patients with relapsed AML.