Twenty-five patients treated surgically for gastrointestinal carcinomas (16 rectum-sigmoid colon, 6 colon, 3 stomach) were investigated by immunoscintigraphy (IS) using 111In-labelled anti-CEA antibody (mouse monoclonal F023C5) F(ab')2 fragments in order to visualize questionable abdominopelvic recurrences (excluding the liver). Fifteen (60%) patients showed a rise in serum CEA levels above 5 ng/ml. Planar scans and emission computed tomographic (ECT) imaging were carried out without reference to the results of computed tomographic (CT) scans and gastrointestinal endoscopic examinations done 1-2 weeks before IS. Final diagnoses were based on biopsies and autopsies (13 cases) or on follow-up findings over at least 2 years (12 cases). Sixteen patients had a final diagnosis of recurrent malignant disease in the extrahepatic abdomen or pelvis. Of these, six were correctly diagnosed by both IS and conventional diagnostic procedures, six by IS only and two by conventional methods only. Two tumour recurrences remained undetected by both diagnostic approaches. However, five tumour recurrences were detected by IS more than 4 months earlier than by any other diagnostic procedures performed during clinical follow-up. Of the nine disease-free patients (disregarding the liver) three were correctly identified by both IS and the other diagnostic methods, four by IS only and two by conventional diagnostic procedures only. Overall sensitivity (75%) and specificity (89%) of 111In-IS were higher than figures obtained using CT scanning and endoscopy (50% and 78% respectively). True positive IS was observed in 6/15 (40%) CEA seropositive patients and in 6/10 (60%) CEA seronegative patients.(ABSTRACT TRUNCATED AT 250 WORDS)