An enzyme immunoassay for the quantitative measurement of CA 19-9 in serum or plasma (EDTA or heparin plasma). The DRG TM-CA 19-9 ELISA Kit is a solid phase enzyme-linked immunosorbent assay (ELISA) based on the sandwich principle. The microtiter wells are coated with a monoclonal [mouse] antibody directed towards a unique antigenic site of the CA 19-9 molecule. An aliquot of patient sample containing endogenous CA 19-9 is incubated in the coated well with assay buffer. After a washing step a second incubation follows with enzyme conjugate, which is an anti-CA 19-9 antibody conjugated with horseradish peroxidase. After incubation the unbound conjugate is washed off. The amount of bound peroxidase is proportional to the concentration of CA 19-9 in the sample. Having added the substrate solution, the intensity of colour developed is proportional to the concentration of CA 19-9 in the patient sample.