Purification of radiolabeled carcinoembryonic antigen (CEA) preparations by affinity chromatography with anti-AG bound to Sepharose was attempted, since an immunological similarity between AG (alpha 1-acid glycoprotein) and a portion of CEA had been noted. When 125I-CEA was purified in this manner, the fraction which did not bind to the column showed decreased reactivity with either anti-AG or anti-CEA. The retained fraction showed enhanced reactivity with both anti-AG and anti-CEA. The yield of purified CEA increased when the CEA preparation was allowed to react with the anti-AG column overnight. Purification of CEA from tumor tissue was performed by affinity chromatography. A perchloric acid (PCA) extract from cancer tissue was mixed with antiserum against CEA to give an immune complex, and a CEA-reactive fraction obtained by PCA extraction. The CEA-reactive fraction was eluted from a Sephadex G-200 column, and final purification was by anti-AG chromatography. When purified CEA was applied to a Sephadex G-200 column with carrier protein after labeling with 125I, the eluted radioactivity was found only in the 180,000 dalton fraction. Almost all the radioactivity was precipitated from the labeled protein by either anti-AG or anti-CEA. Purification of CEA is possible by affinity chromatography with anti-AG bound to Sepharose.