Immunoscintigraphy with CYT-103, an 111indium-labelled immunoconjugate of B72.3, was evaluated in 10 patients before surgery for suspected or biopsy-proven primary colorectal cancer. The imaging results were compared with computed tomography (CT) findings at surgery, histopathology and immunohistochemistry. There were no adverse reactions following the administration of 1.0 mg 111In-CYT-103. Surgical and pathological findings identified 15 sites of disease (10 primary and five metastatic) and all but one lesion (severe dysplasia) were malignant. CT detected nine of 14 sites of malignancy compared to 12 as identified by immunoscintigraphy. It failed to detect two primary lesions and one case of peritoneal metastasis, all of which were imaged by CYT-103. Both imaging modalities failed to detect two of three cases with lymph node metastases and the dysplastic lesion (true negatives). The results indicate that 111In-CYT-103 imaging exhibits high sensitivity and specificity in the detection of primary and secondary lesions in patients with colorectal cancer.