Sixty-four patients with recurrent or metastatic colorectal cancer underwent radioimmunoguided surgery (RIGS). Thirty patients (Group A) were preoperatively injected with radiolabeled monoclonal antibody (MAb) B72.3, a whole IgG1 that reacts with tumor-associated glycoprotein (TAG-72) antigen. Thirty-four patients (Group B) were given monoclonal antibody FO23C5, an F(ab')2 which reacts with the carcinoembryonic antigen (CEA). The use of F(ab')2 antibodies ensured a lower time interval from the preoperative injection of the radiolabeled MAb to surgery. This interval was 22.7 days for Group A patients and 10.9 days for Group B patients. The correct RIGS identification of tumor sites occurred in 80.4% of Group A patients and in 92.6% of Group B patients. Additional information capable of modifying surgical strategy was obtained in 23.3% of Group A patients and in 8.8% of Group B patients. This difference was due to the different patterns of biodistribution and pharmacokinetics of the two MAbs. Although FO23C5 yields an improved diagnostic resolution for macroscopic tumor sites, we believe that B72.3 or other whole IgG1 should be the first choice for RIGS in recurrent or metastatic colorectal cancer patients.