Considerable progress has been achieved during the last two decades in the use of radiolabeled tumor-selective monoclonal antibodies in the diagnosis and therapy of cancer. The concept of localizing the cytotoxic radionuclide to the cancer cell is an important supplement to conventional forms of radiotherapy. In theory the intimate contract between a radioactive antibody conjugate and a target cell enables the absorbed radiation dose to be concentrated at the site of abnormality with minimal injury to the normal surrounding cells and tissues. A variety of approaches and combinations of this strategy are now being pursued. This synopsis attempts to summarize the theoretical and biological basis for radio-immuno-therapy (RIT), and to review present efforts to further develop this treatment. Some of the critical issues in RIT are highlighted, and novel ways of improving the therapeutic indices of these radiopharmaceuticals are outlined. The attention is focused on the results obtained in clinical trials employing RIT. Encouraging complete response rates have recently been reported in patients with non-Hodgkin's lymphoma resistant to combination chemotherapy. More modest results have been obtained in patients with solid cancers. The promises and hurdles in creating tumor-selective radiolabeled antibodies for cancer therapy are discussed, and prospects for further improvements are presented.