A novel substance located in the chief cells of human and rat gastric mucosa, which was detected immunologically by either polyclonal or monoclonal antihuman fibronectin (FN) antibodies, is reported. All three polyclonal antihuman FN antibodies used in this study reacted immunohistologically exclusively with the chief cells. Monoclonal antibody against C-terminal peptide or cell binding peptide reacted clearly with the human chief cells, but monoclonal antibodies against FN N-terminal and midmolecule failed to react with the cells. Western blot analysis of the rat gastric mucosal extract with polyclonal antihuman FN antibody showed that this substance has a molecular weight of about 70,000 Da. Therefore, this substance appears to be a fragment containing the C-terminal peptide of whole molecule FN and thus in the present study is named FN-related substance (FNRS). In a further study with ethanol-induced ulcer model of the rat, the physiological significance of FNRS was examined. The FNRS decreased remarkably, in a dose-dependent manner, in the fundic mucosa of the rats that ingested ethanol. The FNRS appeared to be associated with development of mucosal damage and repair, subsequently playing, in part, an important role in the gastric mucosal protection mechanism.