A radioimmunoconjugate consisting of murine monoclonal antibody (M195) and bismuth 213 (Bi213). Monoclonal antibody M195 binds to CD33, a surface antigen expressed by myelogenous leukemia cells. Bi213 is an isotope that emits short-ranged high-energy alpha particles. This radioimmunoconjugate selectively delivers alpha particle-mediated cytotoxicity to leukemic cells, thereby limiting the exposure of normal tissues to ionizing radiation.