[en] Radioimmunoassay of carcinoembryonic antigen (CEA) was studied fundamentally and clinically by means of Zirconyl-Gel method. Sample was prepared by isolating plasma. At first, CEA was measured by indirect assay involving dialysis after extraction of perchloric acid (PCA). In the samples with CEA value of more than 20 ng/ml, the value was measured by direct assay without the process of extraction by means of ''Roche'' kit. In this assay, there were few quantitative errors by operators, there were scarcely differences between direct assay and indirect assay, and in addition, the assay was found to be reproducible. From these results, it was demonstrated that the assay and the kit were very useful clinically. However, in the indirect assay, CEA indicated higher values than real one in the cases of high values in ionic strength and pH, and on the contrary, it indicated lower values than real one in the cases of low values in ionic strength and pH. Although an increase of CEA value was clinically observed in some benign diseases, the value was relatively low. From the fact that high CEA of more than 10 ng/ml was observed in malignant tumors, it seems to be very significant to determine the metastatic time and the therapeutic effects by means of measuring CEA in plasma. (Kanao, K.)