A sensitive, specific, noncompetitive, sandwich-type radioimmunoassay for carcinoembryonic antigen (CEA) has been developed in our laboratory, which can be performed conveniently. The assay involves two monoclonal antibodies, selected for high affinity and specificity and also for reaction against antigenic sites on CEA that are distal from each other. One of these antibodies was labeled with ¹²⁵I and the other was conjugated covalently to biotin. Polystyrene tubes were conjugated covalently to avidin. These tubes represent a rapid, simple method for separating the CEA-bound antibody from the free antibody. The biotin-antibody-CEA-¹²⁵I-labeled antibody complexes bind to the tubes and CEA concentration is directly related to counts per minute. This assay can detect the CEA at a concentration of 0.22 μg/L in serum.