Intraoperative radioimmunodetection is a new staging technique for epithelial neoplasms. After intravenous injection, a radioimmunoconjugate compound (monoclonal antibody and isotope) attaches itself to the antigens expressed on the surface of neoplastic cells, causing them to emit gamma rays. Radioactivity is detected at the presurgical stage by immunoscintigraphy and intraoperatively with a gamma-detecting probe (GDP). In our experience, intraoperative tumor-to-normal-tissue (T/NT) GDP ratio counts > or =3:1 were considered positive radioimmunoguided surgery (RIGS) findings. We studied 52 colorectal cancer patients, 48 with primary lesions and 7 with relapses. Positive RIGS findings were obtained in 33 patients whose lymph node assessments were characterised by eight false positive identifications. In 10 patients, RIGS data yielded a more accurate staging of the disease. Unreliable RIGS findings were recorded in 15 patients: four injected with monoclonal antibody (MAb) B72.3 labeled with Iodine-125; five with MAb anti-carcinoembryonic antigen (CEA) labeled with 99mTechnetium (Tc); two with MAb B72.3 labeled with Indium-111; and three with MAb B72.3 labeled with Iodine-131 plus interferon-beta. In analysis, our experience did not permit definitive evaluation of the method's diagnostic and staging accuracy.