The epithelial cells lining the oxyntic mucosa in the stomach "corpus" were identified, localized, and counted in 2-month-old male C57BL-6 mice, using glutaraldehyde-formaldehyde fixation and osmium tetroxide postfixation for studies in the light microscope (LM) while adding tannic acid to the fixative and postfixing in ferrocyanide-osmium for studies in the electron microscope (EM). The cells form a single epithelium, which invaginates into blind tubular units. Each unit is divided into four successive regions: pit, isthmus, neck, and base. On the average, a unit contains 194.2 cells. The cells have been classified into three groups totaling 11 types, listed with their mean number per unit. The first group is composed of three well-characterized cell types, each restricted to a region: (1) 37.0 surface mucous cells, hereafter called pit cells, in the "pit" region, (2) 12.6 mucous neck cells, simply called neck cells, in the "neck" region, and (3) 67.4 zymogenic cells in the "base" region. The second group is also composed of three well-characterized cell types, distributed over the four regions: (1) 26.0 parietal cells, (2) 13.2 entero-endocrine cells, and (3) 0.6 caveolated cell. The third group consists of five cell types, which have been little or not characterized in the past. Four are located in the "isthmus" region and show EM features indicative of immaturity, that is, a nucleus with mainly diffuse chromatin and large reticulated nucleoli, and a scanty cytoplasm rich in free ribosomes: (1) 17.2 cells are the least differentiated in the epithelium; they are devoid of secretory granules and accordingly named granule-free cells, (2) 10.0 cells contain a few dense secretory granules smaller than, but otherwise similar to, those in pit cells; they are referred to as pre-pit cells, (3) 1.8 cells possess a few marbled secretory granules that often exhibit a pale core and are smaller than, but otherwise similar to, those in neck cells; they are called pre-neck cells, (4) 0.6 cells display long microvilli and/or small canaliculi similar to those in parietal cells; they are named pre-parietal cells, and (5) 5.6 cells restricted to the base region are characterized by secretory granules with features intermediate between those of neck and zymogenic cells; they are named pre-zymogenic cells. The observations suggest the following hypothesis on cell filiation.(ABSTRACT TRUNCATED AT 400 WORDS)