The safety and clinical utility of repeated administrations of 111In-ZCE 025 were evaluated in 25 patients who have undergone colorectal carcinoma resection. Fifteen patients were clinically and radiologically free of recurrences and asymptomatic while 10 had rising CEA and/or symptoms. We repeatedly imaged the patients following intravenous administrations of 40 mg ZCE 025, every 4 to 6 mo. Side effects occurred in 16% of patients who received two or more infusions. Sixteen lesions were detected by immunoscintigraphy in 11 patients who were free of disease by CT scans or other imaging modalities. Ten recurrences were surgically confirmed in seven patients. Radiographic and clinical follow-up confirmed the remaining 6 Mab-positive lesions. Elevated human anti-mouse antibody (HAMA) titers were detectable in the sera of 30% and 64% of patients following the 1st and 2nd Mab injection respectively, but did not interfere with successful immunoscintigraphy nor correlated with the occurrence of side effects. This study suggests that a negative Mab scan indicates that a patient will remain free of recurrence; conversely, a positive scan was associated with recurrences of disease.