Gastric pepsinogens were studied by immunoenzymologic and immunohistochemical methods in non-cancerous adult gastric mucosa, in fetal stomach, and in gastric carcinomas. In noncancerous adult mucosa, immunoenzymologic methods showed that pepsinogen II (previously called Pg I-II) was found mostly in fundic or mediogastric extracts, whereas Pg IV was predominant in antropyloric extracts. Pg II was localized by immunofluorescence techniques in the chief cells of deep glands found near the muscularis. Pg II was localized by immunofluorescence techniques in the chief cells of deep glands found near the muscularis. Pg III was present in the superficial zone of deep glands and Pg IV in the surface epithelium and in the superficial glands of the mucosa. In fetal stomach, only Pg IV (which we called the "fetal pepsinogen") was identified by standard immunoelectrophoresis. It was localized by immunofluorescence in the superficial epithelium. However, bidimensional immunoelectrophoresis and immunofluorescence did reveal low levels of Pg II and Pg III by the 8th or 9th week of ontogenesis. Pg II and Pg III were localized in the rudimentary glands near the superficial epithelium. Important differences were noted when gastric carcinomas were compared with normal mucosa. Pg II or Pg III were often absent in the tumor extracts, but Pg IV was regularly and, at times, exclusively identified regardless of the location of the tumor. By immunofluorescence Pg IV was often the only pepsinogen that was characterized in dedifferentiated cancerous cells; this was also true of the calciform cells of some metaplastic gastric mucosa. Pg III was observed rarely, and Pg II never, in cancerous cells. The biochemical modifications of the cancerous tissue closely resemble those of fetal tissue.