The trefoil peptides pS2 and human spasmolytic peptide are putative growth factors, particularly associated with mucus-producing cells of the gastrointestinal tract including those of the stomach. The receptor for transforming growth factor alpha (TGF alpha) takes its name from one of its alternative ligands, epidermal growth factor, and is called the epidermal growth factor receptor. Although there is immunoreactive epidermal growth factor in the stomach, it is TGF alpha and the epidermal growth factor receptor that are abundant. Immunolabelling at electron microscope level allows for subcellular localisation of antigens; pS2 and human spasmolytic peptide co-localise to cytomembranes, including the Golgi apparatus, and thecae of surface/pit mucous cells. TGF alpha is abundant on the membranes of tubulovesicles of parietal cells and is also present in chief cells: in mucous producing cells it can be detected but not in association with mucous. The distribution of the epidermal growth factor receptor mimics that of TGF alpha but with preferential clustering on the basolateral membranes of gastric cells. The trefoil peptides are associated with healing and probably act, together with mucus, to protect the gastric mucosa and maintain a viable environment. TGF alpha, transduced via the epidermal growth factor receptor, inhibits gastric acid secretion, thus aids the trefoils in the maintenance of a gastric microenvironment conducive to healing after damage. TGF alpha, however, is also a potent mitogen; while this property plays a vital part in repairing mucosal defects, if this peptide or indeed its receptor are overexpressed, the result can be neoplasia.