Gnotobiotic piglets were challenged at 3 days of age with Helicobacter (Campylobacter) pylori and then transferred to conventional housing at 24 days of age. Piglets were sacrificed 43-45, 57-60, and 87-90 days after challenge. H. pylori was recovered from the stomachs of all piglets challenged except for two piglets sacrificed 45 days after challenge. H. pylori was not recovered from uninfected control piglets. In addition to H. pylori, Lactobacillus and Bacillus species were recovered from conventionalized piglets. Serum from infected piglets contained H. pylori-specific IgG, IgM, and IgA. IgM titers peaked at 14 days after challenge and then declined. IgG and IgA titers continued to rise over the course of the experiment. Piglets in both uninfected control and challenged groups demonstrated lymphofollicular gastritis, but the gastritis was more severe in challenged piglets.