Little data exist regarding the activity of gastric parietal and G cells in the very immature infant. Therefore, we have examined the developing human stomach for the presence and location of parietal and G cells, by using both standard histological methods and antibodies to the H+/K(+)-ATPase (proton pump), intrinsic factor and gastrin in 25 fetuses (ranging from 13-28 weeks) and in 5 infants (2-21 weeks). Parietal cell activity was noted in the body, antrum and pyloric regions in all the fetal specimens examined. However, this activity was much more limited in the infant specimens. Gastrin immunoreactivity was noted in all specimens from 18 weeks of gestation onwards; this activity was located solely in the antral and pyloric region. These results indicate that the human fetus has the potential to produce gastric acid, intrinsic factor and gastrin from the middle of the second trimester.