Abstract: A monoclonal antibody is described, having a specific reactivity with a cancer-associated antigen selected from pancreas cancer-associated antigen, intestinum crassum cancer-associated antigen and hepatoma-associated antigen and has the following characteristics:(1) it exists in a blood serum of pancreas cancer, intestinum crassum cancer and hepatoma patients;(2) it comprises glycoproteins;(3) it has a molecular weight of about 700,000 to 1,500,000, as measured by gel-filtration and determined by comparison with that of a known protein;(4) it has a maximum absorption of 2800 nm when dissolved in 0.1 M acetic acid buffer solution (pH 4.5); and(5) it is released at least in a cultured supernatant medium of establihed cell line of COLO-201, TE-1, TE-2, TE-3, NRC-12, MKN-45 and KATO III. The monoclonal antibody is utilized as a reagent for detecting pancreas cancer, intestinum crassum cancer and hepatoma. A process for preparing the monoclonal antibody and a process for preparing the reagent are also disclosed.