A carbohydrate antigenic determinant (CA 19-9) which monoclonal antibody (1116 NS-19-9) recognizes was found by Koprowski et al in 1979. Recently DelVillano et al have developed a forward sandwich radioimmunoassay (RIA) for quantitative measurement of CA 19-9. This CA 19-9 RIA Kit (CENTOCOR) was examined both experimentally and clinically. In the basic study, the standard curve obtained from this assay showed liniality, and the reproducibility of Intra-assay and Inter-assay was good. In the clinical study, using 37 U/ml as the reference value, the CA 19-9 RIA provided excellent sensitivity for pancreatic cancer (84%) and cancer of the biliary tract (69%), and excellent specificity, even among patients with benign gastrointestinal disorders. Moreover, CA 19-9 was suggested to be useful for monitoring of pancreatic cancer. Thus, the data indicate that CA 19-9 RIA can be a useful tumor marker for pancreatic cancer.