A number of novel strategies have been employed to enhance the degree and specificity of tumor-targeting by antibody-based molecules. These fall into three overlapping categories. The augmentation of the interaction between the antibody and target antigen (e.g., through increased affinity or valence), the enhancement of targeting specificity (e.g., by altering the clearance of unbound antibody or radiolabel), and the selection of radioisotopes and labeling methods which are more stable in vivo or better suited to the target application.