Accurate assessment of the extent of primary and metastatic large bowel cancer is critical to surgical decision making and to providing reliable prognostic information. This prospective study compared external gamma camera images and an intraoperative hand-held gamma detecting probe for detection of radiolabelled monoclonal antibody (B72.3) in 28 patients with primary and metastatic large bowel cancer. Fourteen patients received 0.2 to 20 mg (2 or 5 mCi) 111indium-labelled monoclonal antibody B72.3 followed by whole body imaging scan with an external gamma detector/camera on two occasions 24 h apart within 7 days after injection. Fourteen patients received 1.0 mg (2.0 mCi) 125iodine-B72.3 followed by intraoperative probe evaluation 2-3 weeks postinjection. Mean patient ages for the two groups were 60 years (range 28-75 years) and 63 years (range 43-77 years), respectively. Disease sites were primary in the large bowel in six patients and primary as well as metastatic in 22 patients. External scanning detected 111indium-B72.3 uptake in 1/5 primary lesions, 1/7 hepatic and 1/3 extrahepatic sites. The intraoperative gamma probe localized disease in 1/3 primary lesions, 7/11 hepatic and 3/3 extrahepatic sites. The intraoperative gamma probe had a sensitivity of 71% for detection of metastases compared with a 20% sensitivity using the external gamma scan method (P = 0.03). 125iodine-labelled B72.3 influenced the extent of the operative procedure in 4/14 (29%) patients; immunolocalization with external gamma detection did not alter the operative procedure in the 14 patients studied.(ABSTRACT TRUNCATED AT 250 WORDS)