Isolation of a spiral-shaped bacterium closely related to Helicobacter pylori from the cat stomach made it possible to investigate new small animal models of gastric infection. Pure cultures of this bacterium, provisionally named "Helicobacter felis," were fed to germ-free mice. The organism colonized the stomach in large numbers in mucus and deep in the gastric pits and showed the same gastric trophism found with H. pylori. Significant histopathology was seen in all H. felis-infected mice. At 2 weeks postinfection, an acute inflammatory response was seen composed primarily of eosinophils and neutrophils. At 3 weeks, the polymorphonuclear response was more pronounced with large numbers of neutrophils in some areas forming small microabscesses. Lymphocytes also increased in number. By 8 weeks, several relatively large lymphoid nodules were present in the submucosa. Multiple small microabscesses were still present in the pyloric mucosa. This is the first animal model of bacterial gastritis to be described that shows progression from acute inflammation to persistent acute on chronic inflammation (active chronic) as is seen in human infection with H. pylori.