Tumor associated monoclonal antibodies are potential therapeutic agents as selective carriers of cytotoxic agents to malignant cells. We are testing this hypothesis in several animal model systems. The cytocidal agents being employed are various radionuclides. Their relative efficacy when conjugated to monoclonal antibodies is being assayed and compared to that of monoclonal antibodies alone or conjugated to toxins. The several radionuclides chosen for study span the range of radionuclidic properties available. Thus, Copper-67 is a weak, short range beta emitter. Yttrium-90 is a long range, high energy beta emitter and Bismuth-212 and Bismuth-213 are short range alpha particle emitters. Most recently, we have obtained a radionuclidic generator for Bismuth-213. We have shown that this source can be successfully used for labeling antibodies that have been chemically modified by linkage to the cyclohexyl-DTPA ligand. Good labeling efficiencies were measured. Studies like these are providing for human medicine a basis for design of rational therapies of malignancies by selectively targeting cytocidal agents to tumors, as well as metastases and will in addition allow improved diagnostic imaging of malignancies.