Although carcinoembryonic antigen (CEA) has been the subject of interest for many investigators for over 20 years, many questions still remain unanswered concerning the CEA molecule. These include the ultimate clinical potential of CEA as a tumor marker (specificity, sensitivity, distribution), the biological role of CEA, and the genetic control of CEA synthesis. Initially, much of the work with CEA concerned its physicochemical and immunochemical properties, as well as of cross-reacting molecules, and methods of serologic detection of CEA. More recent studies have focused on cloning of the CEA gene and genetic control of CEA production. An extensive literature exists concerning the role of serum CEA assays and their potential value in determining the prognosis and monitoring of patients affected with cancers of various organs. Despite extensive research into the biology of CEA, few papers deal with the application of CEA immunohistochemistry and immunocytochemistry as concerns normal cellular development, the degree of tumor anaplasia, and the various diagnostic problems of surgical pathology. There has also been a great deal of interest in the utility of both polyclonal and monoclonal antibodies to CEA both in the radioimmunolocalization and potential therapy of CEA-producing tumors. This review summarizes the past and current findings of the clinical applicability of the serum measurement of CEA and examines the status of radioimmunolocalization of tumors as a basis for effective antibody targeted immunotherapy in the future.