The distribution and type of immunocompetent cells were investigated in rat oral mucosae using immunocytochemistry and enzyme histochemistry, focusing on histological structures. We used two antibodies, OX6 and ED1, which recognize the rat la-antigen and macrophage/monocyte lineage, respectively. Enzymatic histochemistry for acid phosphatase (ACPase) activity and adenosine triphosphatase (ATPase) activity was also employed to identify macrophages and Langerhans cells, respectively. Many OX6-immunopositive cells, dendritic or irregular in shape, were recognizable in the lamina propria of oral mucosae: some cells extended their dendritic processes into the epithelial layer of the buccal and sublingual mucosae. Dendritic cells within the epithelium showed intense ATPase reaction, indicating they could be categorized as Langerhans cells. A small number of ED1-positive cells existed in the lamina propria, but none were present in the epithelial cell layer. Double staining either with OX6 and ED1 or OX6 and ACPase made it possible to divide the immunocompetent cells in the lamina propria into three types: the OX6-positive cells without ED1 or ACPase-reaction, the OX6-negative cells with ED1 and ACPase-reactions, and the OX6-ED1/ACPase-co-expressing cells, each of which possessed characteristic ultrastructural features demonstrated by immunoelectron microscopy. Taking these findings together with previous reports, these three types of cells were regarded as a dendritic-like cell, a macrophage without antigen-presentation ability, and a macrophage with antigen-presentation ability, respectively. There were regional differences in the distribution and density of these immunocompetent cells; they were densely distributed in order of the buccal and sublingual mucosae, the palatal mucosa, and the dorsal surface of tongue. The region-specific distribution and density of the immunocompetent cells might be due to the histological. structure of each oral mucosa, suggesting the presence of different immune-defense systems among each portion of the oral mucosae.