A '3'l-labelled anti-CEA monoclonal antibody (MAb) (F023C5) was used in a preclinical in vivo study in an animal model and in a clinical study in patients with colo-rectal carcinoma. Preliminary screening on different tissue was performed with immunoperoxidase staining, which showed a good specificity of the antibody for gastric and colo-rectal carcinoma. Initially immunoscintigraphy was performed as a pilot study at the National Cancer Institute of Milan on 51 patients with 64 localizations of colorectal cancer. Serial scans were performed after injection of 2 radioactive reagents, namely l3Il and "'In. In vivo pharmacokinetics of the compound were studied. High sensitivity (8OYo) and high specificity were obtained in detecting local recurrences and abdominal metastases whereas a low positivity rate was obtained in detecting liver metastases. Our data were confirmed by preliminary results of the multicentric trial organized by the Italian National Research Council on 300 patients with about 700 localizations of CEA producing tumors.