The size and location of Peyer's patches (PP) in the jejunum and ileum and their composition of lymphocyte subsets (B, CD2+, CD4+, CD8+) have been studied in conventional and gnotobiotic Göttingen minipigs. Each PP in the small intestine remained at the same site and was of comparable length between 2 and 12 months of age. In 1.5-month-old conventional minipigs the histology of the compartments differed between the continuous PP in the terminal ileum (ilPP) and the discrete PP in the jejunum (jejPP). No such difference was seen in gnotobiotic or in 12-month-old animals. The composition of lymphocyte subsets showed striking differences with significantly more B and less T, CD4+ and CD8+ cells in ilPP in 1.5-month-old minipigs in comparison with 12-month-olds. Mesenteric lymph nodes and jejPP displayed a typical pattern of lymphocyte subsets. The size of the lymphocyte compartments in PP and their cellular composition depends largely on age and microbial influences from the gut lumen, which might be of major importance for studies on the function of the gut-associated immune system in the pig.