The successful use of radionuclides for tumor therapy depends to a major extent on the ability to achieve a high concentration of radioactivity in the tumor relative to other radiosensitive organs not involved by tumor, such as bone marrow, intestinal mucosa, liver, and kidneys. Techniques designed to achieve such differential localization of the radionuclides include the use of radiopharmaceuticals that enter specific metabolic pathways unique to certain tumor types; radiolabeled antibodies that attach to tumor-associated antigens present on tumor cell surfaces; heterologous antibodies that attach to tumor-associated antigens present on tumor cell surfaces and which are then identified by radiolabeled antibodies directed against the species in which the original, unlabeled antibody was made, and radiolabeled compounds injected regionally at the tumor site. Although both clinical and experimental evidence on the use of radionuclides for tumor therapy is encouraging in preliminary studies, extensive further research needs to be done in this area to insure the clinical efficacy of radionuclides for tumor therapy.