Carcinoembryionic antigen (CEA) a glycoprotein extracted from colonic cancer tissue (beta-globulin electrophoretic mobility, sedimentation coefficient 7 to 8S, and mol wt approximately 200,000) can be detected and measured by radioimmunoassay. Clinical evaluations of CEA determination have given the following results: In health: (1) Serum CEA level is not influenced by sex, age, blood type, time of blood sampling, or family history of cancer; (2) serum CEA level is influenced by a history of smoking or inflammatory disease of the bowel, lung, pancreas, and other organs (occasionally, a CEA level as high as 10 ng/ml is noted); and (3) currently, CEA positivity is defined as greater than 2.5 ng/ml, however, 5 ng/ml may be more realistic. In cancer: (1) CEA level may be increased in primary cancer of the gastrointestinal (GI) tract as well as in non-GI neoplasia; (2) the CEA test is not recommended for screening to detect early cancer; (3) serum CEA level depends on the stage of the neoplasia and usually is not influenced by the grade of differentiation; and (4) markedly increased (greater than 25 ng/ml serum CEA values are highly suggestive of metastatic cancer, particularly hepatic metastasis. In biological fluid: The CEA or CEA-like activity can be measured in gastrointestinal secretions. Quantitative studies of CEA levels in such fluids may yield more information than is obtainable from studies of serum. However, this possibility needs more study at present. Therefore, the currently available CEA tests cannot replace any of the now standard diagnostic methods for cancer detection. This use for assessment of therapy in selected patients or for following those known to be a high risk for cancer appears promising in preliminary studies, but clinical value, if any, remains to be determined.