The critical issues in radioimmunotherapy are highlighted, and novel ways of improving the therapeutic indexes of radioimmunotherapeutic agents are outlined. The use of radioactively labeled monoclonal antibodies to treat malignant tumors has been investigated in animals and humans. Radionuclides suitable for labeling antibodies for such use include iodine 125, iodine 131, yttrium 90, rhenium 188, and copper 67. Radiobiological factors to be considered in radioimmunotherapy include the size and density of the tumor and the ability of a radiolabeled antibody to penetrate the tumor nodule. The dose of radiation required to destroy a tumor varies; however, the whole-body dose must not exceed 200 rads to avoid irreversible toxicity to the bone marrow. Despite the theoretical inadequacy of radiation doses to tumors indicated by conventional dosimetry, responses have been observed in animals and humans. More reliable and accurate dosimetric methods are under development. The induction of human antimouse antibodies can alter the pharmacokinetics of radiolabeled antibodies. Improving the therapeutic index of radioimmunotherapeutic agents may be achieved through regional therapy, administering a secondary antibody to improve clearance, combining radioimmunotherapy with external-beam irradiation, using an avidin-biotin conjugate system to deliver the radiolabeled antibodies, and addressing the problem of tumor antigen heterogeneity. Researchers are working to reduce or eliminate the clinical problems associated with radioimmunotherapy. Hematologic malignancies, such as lymphomas, are more likely than solid tumors to respond satisfactorily.