The in vivo use of radionuclide-labeled antibodies to several types of tumor-associated antigens or products has proven to be a reliable method for defining primary, recurrent, metastatic, and occult carcinoma. Specificity for the radioimmunodetection procedure is high; however, the sensitivity varies depending on the size and site of the lesions. The relative roles of polyclonal- and monoclonal-type antibodies are still to be defined, since at this stage of development little difference can be demonstrated in the detection of tumor with either one. The introduction of emission computed tomography should simplify the imaging techniques appreciably and improve the resolution between nontarget and target radioactivity. Satisfactory radionuclide labels with photon energies more compatible with current imaging equipment will enhance resolution and sensitivity.