The serum levels of carcinoembryonic antigen (CEA) and CEA-binding proteins of 68 controls and 170 cancer patients were determined. The CEA levels were determined by a double-antibody radioimmunoassay using radiolabeled CEA, and the CEA-binding proteins were determined by an immunoassay utilizing radiolabeled CEA and polyethylene glycol. The major CEA-binding proteins in serum were anti-blood group antibodies as demonstrated by differential binding of serum proteins from A, B, or O positive individuals to radiolabeled CEA's which were previously shown to carry specific blood group determinants. No statistically significant differences were observed for the binding of control versus cancer patient sera to CEA-A (carrying blood group A1), except, as expected, A negative (O or B positive) individuals gave high binding to CEA-A, while A positive individuals gave low binding to CEA-A. Statistically significant differences were observed for controls versus cancer patients for binding to CEA-Lewisab (CEA-Leab). CEA-Leab-binding activity was higher in females and smokers in the control group, but this distinction was not found in the cancer patients. The high levels of anti-Leab antibodies may be explained in females by exposure to fetal antigens during pregnancies and in smokers or cancer patients by exposure to precursors to blood group substances. The sera of 7% of the patients with colonic carcinoma, 18% of the patients with breast carcinoma, and 23% of the patients with bronchogenic carcinoma bound more CEA-Leab than did the serum of the highest male control. A correlation between CEA-Leab-binding activity and the levels of serum CEA was found for patients with colonic carcinoma but was not significant for the groups with other cancers or the control group. Serial determinations of CEA-Leab-binding activity for 21 patients with bronchogenic carcinoma changed congruently with the serum levels of CEA in 12 cases. The significance of these results in terms of expression of immature blood group antigens, the subsequent production of antibodies against them, and the prognostic value of this response is discussed.