In order to determine the possible effects of the circulating intrinsic factor antibodies (IFA) on gastric morphology and secretory function, four groups of 12 rats each received intravenous injection daily for 8 to 12 weeks of immunoglobulin G (IgG) fractions separated on DEAE-cellulose columns from various sources: (1) sera of patients with pernicious anemia, containing both IFA and parietal cell antibodies (PCA), (2) sera from patients with atrophic gastritis, containing parietal cell antibody only, and (3) and (4) sera of rabbits immunized with semipurified human or rat intrinsic factor (IF). In addition three control groups of 12 rats each received intravenous injections daily for 8 to 12 weeks of either (5) saline or (6) and (7) IgG processed from human or rabbit normal sera. Still another group of 12 rats (8) did not receive any injections whatsoever for the same duration of time. One-third of the rats were intubated biweekly after histamine stimulation and the hourly outputs of CHl pepsin, and IF were determined. At conclusion of the experiments, rats were killed, the mucosal surface and thickness of the mucosa were measured, and parietal cell and peptic cell masses were counted. The control groups showed either progressive growth of the cellular mass in gastric mucosa and increase of the HCl, pepsin, and IF outputs, or no significant changes. In contrast, rats injected with IgG containing IFA to human or rat IF showed a statistically significant thinning of the gastric mucosa, reduction of peptic cells, which are known to secret IF in this species, and corresponding decreases in the ouputs of pepsin and IF. These became reduced by about 50% from initial values, and by 62 or 75%, respectively, when compared to rats injected with normal human or rabbit IgG's. In rats injected with IgG's from pernicious anemia sera, which contained both IFA and PCA, the outputs of IF, pepsin, and HCl decreased signigicantly, as well as the peptic and parietal cell masses. The rats injected with PCA only demonstrated thinning of the gastric mucosa, reduction of parietal cell mass, and a significant decrease of the HCl output. These findings imply an active role of the circulating gastric.