To compare radiolocalization of murine B72.3 (m-B72.3) and mouse/human chimeric B72.3 (ch-B72.3) antibodies, five patients with biopsy confirmed adenocarcinoma of the colon received both radio labeled antibodies 4 or 7 days before laparotomy. Following antibody administration, preoperative gamma camera images showed localization to sites of disease in four of the five patients. Autoradiography of resected specimens showed that both labeled antibodies localized specifically to the tumor with only minimal amounts in normal tissues. Radioactivity from each isotope in biopsy specimens of tumor and normal tissues was quantitated by scintillation gamma counting. Comparison of the percentages of injected activities for each antibody in resected tumor and normal tissue yields tumor to normal tissue radiolocalization ratios of 2.7–13.3 and 0.9–6.3 for murine and chimeric antibodies, respectively. The higher ratios for murine antibody were due to lower normal tissue levels, reflecting its faster clearance from the circulation, whereas the quantitative uptake of labeled antibody was always greater with the chimeric antibody. The chimera to murine antibody ratios in tumor of 1.1–2.7 suggest modest enhancement of tumor localization with chimeric antibody because of its longer half-life. © 1993 IOS Press. All rights reserved.