Developments in hybridoma technology leading to the production of monoclonal antibodies recognizing tumor-associated antigens are providing new approaches for the radioimmunodetection of, and drug targeting to, metastases. These developments are illustrated in a series of studies on the in vivo localization of an antihuman-osteogenic sarcoma monoclonal antibody (791T/36) in human tumor xenografts maintained in immunodeprived mice. 131I-labelled 791T/36 antibody localized specifically in osteogenic sarcomas but not in xenografts of other tumors, such as bladder carcinoma T24, which do not express the antigen identified by this antibody. Developing from these studies, various parameters influencing antibody localization in tumors were examined including the kinetics of antibody uptake, the relationship between tumor size and antibody binding, and the site of antibody deposition. This provides a basis for considering the potential of antitumor monoclonal antibodies for targeting antitumor agents. Of particular importance here is the observation that antibody is principally located at the periphery of tumors since this will influence the population of cells within a tumor which can be attacked by antibody-drug or antibody-toxin conjugates. Experiments with human tumor xenografts demonstrate tumor localization of radioisotope-labelled 791T/36 monoclonal antibody. Tumor localization by external gamma camera imaging of osteogenic sarcoma xenograft-bearing mice was also demonstrated. These studies illustrate the potential of antitumor monoclonal antibodies for imaging primary and metastatic tumors. This approach is further emphasized by the radioimmunodetection of primary and metastatic colorectal carcinomas.