Markers specific for various lung cells are useful for studies of cellular differentiation and function. We have produced monoclonal antibodies that bind to isolated rat type II alveolar epithelial cells in an ELISA. Two such antibodies, 2C1 and 3F9, specifically labeled type II cells and nonciliated bronchiolar cells by indirect immunofluorescence of rat lung. A third antibody, 2A3, recognized isolated type II cells by ELISA and immunofluorescence, but did not bind to sections of whole lung. Further immunofluorescence studies on adult rat tissue showed that neither 2C1 nor 3F9 labeled other lung cells or cells in kidney, small intestine, brain, or trachea. The antigen or antigens recognized by 2C1 and 3F9 was not detectable at day 15 of fetal lung gestation but was detectable by day 21. Immunofluorescence studies carried out on 0.5-microns frozen sections of lung tissue demonstrated that both 2C1 and 3F9 bound to cell surface antigens, which are expressed in a highly polarized fashion on the luminal surface of the alveolus and bronchiole. The rat cell line, L2, which displays some similarities to type II cells, did not display positive immunofluorescence to 2A3, 2C1, or 3F9. The antibodies 2C1 and 3F9 are distinct from and apparently more specific than previously described monoclonal antibodies raised to rat type II cells. Alveolar type II and nonciliated bronchiolar cells share several common features. Both cell types contain the surfactant apoprotein SP-A, proliferate in response to lung injury, develop in the late stages of gestation, take up and catabolize platelet-activating factor, contain high levels of cytochrome P-450, and can be induced to form tumors in response to chemical carcinogens. The recognition of highly specific surface antigen(s) on both nonciliated bronchiolar cells and type II cells demonstrates yet another characteristic shared by the two cell types.