In this paper we summarize the investigations performed by our group utilizing an anti-CEA monoclonal antibody (F023C5) labelled with different radionuclides in humans. Since 1983 radioimmunoscintigraphy (RIS) was performed on 51 patients with 64 localizations of colo-rectal carcinoma (pilot study). A multicenter clinical trial in a large number of patients (509 pts of which 284 with gastrointestinal cancer) was subsequently carried out in collaboration with ten nuclear medicine centres. High sensitivity and specificity values were obtained by these studies and many unsuspected lesions were recorded. In order to better define the clinical role of RIS, a prospective study was performed on 59 patients with suspected local relapses of colo-rectal cancer. A comparative evaluation of RIS, CT scan, US and MRI was done. RIS and MRI had the highest accuracy (86%) followed by CT scan (68%) and US (54%).