Serial CEA radioimmunoassays have been performed on patients receiving radiation therapy preoperatively or as treatment of recurrent or metastatic colorectal cancer. Pretreatment circulating CEA titers which were elevated showed a significant decrease with accumulating doses of irradiation, indicating that the bulk of CEA-producing tumor was within the radiation therapy portal. The decrease of circulating CEA with preoperative radiation therapy was short-lived and suggested that surgical resection should be performed within 8 weeks of irradiation. Serial CEA titers are useful as an adjunct to other clinical, laboratory and radiologic data in formulating patient management decisions.