In order to evaluate some of the key factors that may allow the optimization of radiolabeled monoclonal antibodies for use as diagnostic and therapeutic tools to detect and treat human neoplasia, we compared the biodistribution of the anti-lung-tumor monoclonal antibody KS1/4, labeled with four different radionuclides, in athymic (nu/nu) mice bearing human lung adenocarcinoma. Several radiolabeling methods were used: the first involved coupling a suitable bifunctional chelating agent, such as DTPA, to the KS1/4 monoclonal antibody, followed by binding the radiometal, either 113mIn or 111In. Radioiodination was carried out by the chloramine-T method with 131I, and intrinsic labeling by generating the hybridoma in the presence of 75Se-methionine. An examination of tumors and major organs of mice injected with one of the above radiolabeled KS1/4 MoAbs, and biodistribution at various time intervals up to 96 h post injection revealed that iodination and intrinsic labeling yield the highest tumor uptake. Because of the relatively high deiodination that occurs in vivo, and the high 75Se content in the circulation, the preferential uptake (tumor-to-blood ratio) of these radiopharmaceuticals lags behind the equivalent ratios for the In-labeled MoAb. In the latter group of animals, a second consecutive injection of the labeled MoAb resulted in elevated blood level of radioindium, as well as a corresponding decrease in tumor-to-blood ratios.