Monoclonal antibodies against a range of surface markers were used to localize T cells, macrophages and dendritic cells (DC) in sections of rat trachea and peripheral lung, employing the immunoperoxidase technique. A population of Ia-bearing cells with characteristic DC morphology was identified within the tracheal epithelium, closely associated with the basement membrane, and Ia+ dendritic processes from these cells penetrated the epithelium reaching the overlying fluid layer. A second DC-like population, also Ia+ but differing from the airway DC in expression of other markers, was identified within the alveolar septal walls. Both types of DC were intimately associated with populations of pleiomorphic heterogenously staining macrophages. In addition, a large population of T lymphocytes was identified within the alveolar septa; the lymphocytes occurred as single, isolated cells, distributed randomly throughout the lung parenchyma.