The effect of deprivation of Peyer's patches (PP) on transport of lymphocytes through intestinal lymph and intestinal mucosal immune responses was investigated in rats. All visible PP in the rat small intestine were excised in order to examine the roles of PP in the intestinal lymphatic system and mucosal immune responses of the intestine. Two weeks after the experimental excision of PP, lymphocyte transport in intestinal lymph was significantly decreased in PP-excised rats without significant changes in lymphocyte subsets as compared with sham operated control rats. Lymphocyte subsets as determined morphometrically in the intestinal mucosa showed no significant alteration in PP-excised rats. There was a significant decrease in the number of immunoglobulin A (IgA) containing cells in the intestinal mucosa of PP-excised rats, while IgM and IgG containing cells showed no statistically significant changes in number. Conversely, the macrophages in the intestinal mucosa increased in number, suggesting the enhanced accessory functions of these macrophages. Antigen-specific immune response was further studied in PP-excised rats using intraduodenal priming and challenge with cholera toxin (CT). Both the determinations of cells producing antigen-specific antibody in the intestinal mucosa using anti-CT antibody and those of cells secreting anti-CT Ig in the intestinal lymph by enzyme-linked immunospot (ELISPOT) assay showed a significant reduction of CT-specific antibody production in PP-excised rats compared with controls. Peyer's patches appear to have an important role in lymphocyte transportation through intestinal lymph and also in mucosal immune responses.