A radiotherapeutic immunoconjugate comprising a tumor specific monoclonal antibody, or fragment thereof, and an Auger electron emitting radionuclide, wherein the tumor specific monoclonal antibody or fragment is capable of tumor nucleus localization of the radionuclide and method of using it is disclosed. The radiotherapeutic immunoconjugate is administered in a therapeutically effective amount to patients having or suspected of having a malignancy reactive with the antibody or fragment.