Recently, the CA 19-9-MP-Mitsui kit (MP-kit) by a sandwich enzyme immunoassay using two mouse monoclonal antibodies of C192 and C241 for the determination of CA 19-9 was developed by Mitsui Pharmaceutical Co., Ltd.. We carried out basic and clinical studies of MP-kit for the serum CA 19-9 in comparison with the Immunoclone CA 19-9 EIA kit (I-kit) which was provided by Fuji-Rebio Company. It was found that MP-kit appeared to have excellent reproducibility with high sensitivity and specificity for CA 19-9. Furthermore, the coefficient of positive correlation was as high as 0.92 in serum CA 19-9 level between MP-kit and I-kit in the determination of serum CA 19-9 in 67 cases by each of two kits. On the other hand, with two kits, it was found that employment of a cut off value more than 70 U/ml brought a highest diagnostic efficiency for pancreatic and bile duct cancers, while employment of a cut off value of 37 U/ml which was used routinely resulted in a high false positive incidence, particularly in liver cirrhosis or hepatitis. From the results, it is concluded that a pertinent cut off value of CA 19-9 in serum was about 70 U/ml and MP-kit was available for determination of CA 19-9 in serum such as the other known kits on the market for CA 19-9.