A microradioimmunoassay technique is described for detecting carcinoembryonic antigen (CEA) in whole serum. It differs from previous methods in being performed on 0·025 ml of whole serum instead of 5 ml of serum extracted with perchloric acid. The present assay was sufficiently sensitive to detect 85% of carcinomata, localized to the colon, but positive results occurred also with certain non-gastrointestinal cancers, chiefly lung and breast, and certain non-malignant diseases. Many of the latter sera, with the general exception of alcoholic cirrhosis and pancreatitis, gave negative results after extraction with perchloric acid. It is suggested that a direct assay for CEA in whole serum may permit testing of large numbers of sera by laboratories with facilities for radioimmunoassays.