A mouse monoclonal antibody V-715 was raised against fresh colon cancer tissues. Biochemical analysis elucidated that the antigen defined was adenosine deaminase binding protein (ADBP). In colon cancer cell lines, V-715 was positive in 8 out of 16 differentiated cancers and in 2 out of 8 poorly differentiated cancers. In frozen sections, ADBP was expressed in 17 out of 33 differentiated colon cancers, but none of 4 poorly differentiated colon cancers. In normal colon, the expression was observed in epithelium. In gastric cancers, ADBP was expressed in 10 out of 15 differentiated cancers, but weakly or only heterogenously expressed in 2 out of 8 poorly differentiated cancers. In normal gastric mucosa, ADBP was mainly detected in the foveolar epithelium, but was weakly or not expressed in the deep gastric glands. Carcinoid tumors and malignant lymphoma of the stomach did not express ADBP. These results suggest that ADBP may act as a marker of enterocytic differentiation in normal and neoplastic gastrointestinal cells, and might be exploitable in clinical and pathological diagnosis of gastrointestinal cancers.