Carcinoembryonic antigen (CEA) from colonic carcinoma extracts was labeled with radioiodine. The 125ICEA appeared homogenous in a number of systems but showed three components on density gradient ultracentrifugation in CsCl. The component of intermediate density appeared to represent purified CEA and reacted vigorously with absorbed rabbit and goat anti-CEA sera. The most dense component reacted with antiserum to blood group A substance as well as with the two anti-CEA sera. Its reactivity with

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... all three antisera was abolished by the addition of blood group A substance. The purified CEA also reacted weakly with anti-blood group A serum. This reaction was abolished by the addition of blood group substance which, however, had no effect on the reaction of the purified CEA with the anti-CEA sera. It was concluded that some preparations of tumor antigen contain a mixture of blood group A substance and the tumor antigen. The rabbit and goat anti-CEA sera appeared to contain a mixture of anti-CEA and anti-A antibodies. CEA molecules may contain both "CEA-determinants" and A-determinants. A sensitive radioimmunoassay for CEA was developed by using 125I-labeled purified CEA.