The purpose of this study was to analyse the phenotypical distribution of resident cells of the mononuclear phagocyte system in rat salivary glands, and to determine whether isoproterenol induces alterations in macrophage and lymphocyte surface-marker expression. Frozen sections of gland tissues were prepared from five normal rats, and from six rats treated with 20 mg/kg isoproterenol/day for 10 days. A panel of six monoclonal antibodies was used to identify membrane markers associated primarily with monocytes (ED1), mature tissue macrophages (ED2), lymphoid macrophages (ED3), MHC class II (Ia) antigens (OX6), CD5-positive T lymphocytes (OX19), and rat B lymphocytes (OX33). Double-labelling techniques were used to detect the coexpression of ED1/ED2 and OX6/ED2 mononuclear cell markers in the major salivary glands. ED2-positive macrophages were predominant in all three major glands, ranging from 96 cells/0.87 mm2 field in the parotid gland to 165 cells/0.87 mm2 in the submandibular. OX19-positive T lymphocytes were rarely observed in submandibular and parotid glands but represented a distinguishing feature of the sublingual. Moderate numbers of ED3-positive macrophages also were detected in sublingual tissues. In the submandibular and parotid glands, isoproterenol resulted in a decrease in ED2-positive cells, but ED2-positive macrophages increased in sublingual glands with isoproterenol. Isoproterenol resulted in a decrease in MHC class II antigen expression on submandibular and sublingual mononuclear cells but an induction of Ia antigen in the parotid gland. Double labelling revealed that isoproterenol induced coexpression of ED1/ED2 markers on mononuclear cells in the submandibular glands, but ED1/ED2-positive cells were absent from other glands. However, coexpression of MHC class II markers on ED2-positive cells in the sublingual and parotid glands of normal rats was frequently observed, with isoproterenol decreasing coexpression in the sublingual gland and increasing it in the parotid. B lymphocytes were not detected in any of the glands examined. These findings indicate that important differences exist in normal resident mononuclear cell subsets among the major salivary glands of the rat. The differential effects of isoproterenol on inflammatory cells may reflect important differences in local salivary gland immunoregulation. Although salivary gland inflammation induced by isoproterenol does not appear to result from immune mechanisms, the rich population of T lymphocytes and ED3-positive macrophages, and presence of MHC class II antigens, suggest that the sublingual gland may function as an immune organ and have a role in mucosal immunity.