The sera of 212 patients with malignant and non-malignant diseases have been radioimmunoassayed for the presence of the carcinoembryonic antigen (CEA) using 3 different kits produced of Hoffman La Roche, Switzerland (RCK), BY Sorin-IRE, Italy and Belgium (SCK), and by the Istituto Sieroterapico Milanese, Italy (ICK). In the presence of endodermically-derived system carcinomas, the RCK gave more positive results (72.6%) than did the SCK (63.1%) or ICK (56.2%). With regard to other carcinomas, ICK (50.0%) and SCK (47.1%) gave better results than did RCK (30.6%). The results are discussed in terms of clinical usefulness of the CEA assay and as regards reproducibility, procedural advantages, and economical cost of each kit. It is concluded that the CEA assay cannot be used for the diagnosis of gastrointestinal cancers, although it is useful as a measure of "cancerosity" for prognostic purposes. In this sense the double antibody method employed by SCK and ICK is clinically more advantageous than is the perchloric acid extraction-zirconyl phosphate gel precipitation method of RCK.