For the characterization and identification of potential molecular markers of M cells, the follicle-associated epithelium (FAE) of Peyer's patches from the small intestine of rats was analyzed by indirect immunofluorescence and immunogold electron microscopy. The expression of intermediate filament proteins in the FAE was found to be heterogeneous. Immunoreactivity of a specific monoclonal cytokeratin 8 antibody (clone 4.1.18) was selectively found in cells which were additionally characterized by the lack of staining for alkaline phosphatase in their apical membranes. In the FAE, this property is an accepted criterium for the presence of M cells. Further, these cells were found to be devoid of mucin and were thus distinct from goblet cells. The antigen recognized by the clone 4.1.18 is expressed in substantially higher amounts in M cells of Peyer's patches as compared to neighboring epithelial cells or "normal" enterocytes and thus can be employed as an intracellular molecular marker for M cells of Peyer's patches in rats.