The intermediate filaments of the dome epithelium of porcine Peyer's patches were studied by immunohistochemistry. The labelling patterns of monospecific antibodies directed against cytokeratins 8, 18 and 19 differed considerably. About 40% of the dome epithelial cells were intensely labelled by three different anti-cytokeratin 18 antibodies, indicating that large amounts of cytokeratin 18 are present in these cells. In order to verify that these cytokeratin-18-immunoreactive cells were M-cells, uptake studies using fluorescein-labelled yeast particles were performed. Numerous yeast particles were found exclusively in dome epithelial cells that were highly positive for cytokeratin 18, thus representing M-cells. In contrast, the content of cytokeratin 19 in M-cells was lower than that in neighbouring enterocytes. The labelling intensity of cytokeratin 8 did not differ between M-cells and enterocytes. In addition, the absence of vimentin and desmin from the dome epithelium of porcine Peyer's patches was demonstrated. The results show (1) that porcine M-cells differ from enterocytes in the composition of their cytoskeleton, (2) that cytokeratin 18 is a useful marker for detecting porcine M-cells and (3) that this marker directly correlates with M-cell function.