The carbohydrate antigenic determinant (CA 19-9) EIA kit (FUJIREVIO INC) was examined both experimentally and clinically. In the basic study, the standard curve showed linearity from to 240 U/ml, and the intra-and interassay reproducibility was good. Serum levels of CA 19-9 were determined by EIA in 246 cancer patients and 180 patients with benign diseases, and CA 19-9 levels by EIA were compared with those by RIA in concurrent measurement. There was an excellent correlation between the two. The positive of CA 19-9 EIA was almost the same as that of CA 19-9 RIA in cancer patients. False-positive cases by CA 19-9 EIA were somewhat more numerous than those by CA 19-9 RIA in patients with benign diseases, especially liver diseases. However, CA 19-9 assay by EIA is simple and has an excellent reproducibility and accuracy, as does that by RIA. Thus, the CA 19-9 EIA is a very useful assay.