The presence and features of mucosa associated lymhoid tissue (MALT), analogous to Peyer's patches, in the cardia of the lesser curvature of the porcine stomach are described. The gastric mucosa associated lymphoid tissue (gastric-MALT) is histologically distinct from gastric inflammation associated with colonization by normal gastric microflora and experimental bacterial colonization with a human gastric bacterial pathogen, Helicobacter pylori. The gastric-MALT consists of well-demarcated encapsulated and organized lymphoid tissue, intimately associated with overlying gastric epithelium, centered below the muscularis mucosae and drained by efferent lymphatics. Gastric-MALT was identified in all piglets studied including microbially sterile uninfected gnotobiotes; these structures were enlarged with age and local (gastric) antigenic stimulation. Significant (P < 0.05) expansion of the gastric-MALT occurred in H. pylori-infected gnotobiotic piglets. These distinct morphologic features and location in the cardia suggest that lymphoid elements in the gastric-MALT are involved in gastric antigen processing and regional lymphoid maturation, differentiation and proliferation in the stomach.