Radioimmunoscintigraphy (RIS) of colorectal carcinoma with a 111In-labelled anti-TAG-72 monoclonal antibody (CYT-103) has been performed in 24 patients with five primary lesions and 10 suspicious of recurrence (in one of these patients two RIS were made). Histopathological confirmation of the disease and surgical liver examination were carried out in all primary tumours and in 12 possible recurrences. In the remaining eight patients a final diagnosis was established according to the clinical course and other diagnostic procedures. Planar and tomographic scans were obtained at 48 and 72 h postinjection in all patients. All primary tumours were detected by RIS. In the group with recurrences confirmed pathologically the results were nine true positive, two true negative and one false positive in a patient affected only with liver disease. The nine true positive studies corresponded to four positive by computed tomography (CT), four negative by CT and one nonconclusive by CT. Surgical liver examination results were 15 true negative and two false negative. No correlation was found between serum levels of carcinoembryonic antigen or TAG-72 and the detection of the lesions. In 10 patients human anti-mouse antibody (HAMA) levels were studied. In conclusion, RIS with an anti-TAG-72 monoclonal antibody is a useful technique for the study and localization of colorectal tumours, mainly in cases of recurrence, being also indicated in patients with normal TAG-72 serum levels.