Four monoclonal antibodies (GC301, 302, 303, 304) produced by hybridomas obtained from mice immunized with NUGC3 were analyzed by serological assay (Mixed Hemadsorption Assay-MHA) and immunoperoxidase technique. GC301(IgG1) showed tumor restricted reactivity in serological analysis, but it reacted with fibrous interstitial tissues immunohistochemically. GC302(IgG1) was serologically reactive with epithelial tumors such as gastric cancers, colon cancers and gynecological cancers, but not with brain tumors, melanomas and other normal cells. In tissue sections, all gastric cancers, intestinal metaplasia, stomach of fetus and bile duct were stained, but brain and hepatocytes were not. These results indicate that the antigen detected by GC302 is not only differentiation antigen by which gastrointestinal tract could be divided into foregut and midgut origins, but also the new type of oncofetal antigen different from CEA. GC302 would be useful for preoperative detection of gastric cancer in lymph nodes, using radioimmunodetection scans. GC303(IgG1) and GC304(IgG1) were broadly reactive with various cells in serological assay. Immunohistochemically, GC304 reacted with submucosal connective tissue, which was inhibited by collagenase. The results obtained from GC301 and GC304 suggest the possibility of interaction between tumor cells and interstitial tissues.