Monoclonal antibodies against human acid stable trypsin inhibitor (ASTI) was produced by hybridizing SP2/0-Ag14 mouse myeloma cells with spleen cells of Balb/c mouse immunized with ASTI. Western blotting analysis using purified ASTI and urinary proteins exhibited the specificity of the antibodies against ASTI. The distribution and localization of ASTI in adenocarcinoma of stomach were examined using immunohistochemical techniques. ASTI immunoreactivity was detected from the tubular adenocarcinoma, but not from the signet ring cell carcinoma or mucinous adenocarcinoma.