It is known that the skin is the natural route for induction of allergic contact dermatitis (ACD). Because the lung is another potential portal of entry for sensitizing chemicals, studies were performed to evaluate immune responses to haptens deposited in the lung. As a result, a new method for inducing ACD was developed. Intratracheal (IT) inoculation of 2,4,6-trinitrobenzene sulfonic acid (TNBS) led to maximal ear swelling 24 hr after challenge on the ear with 2,4,6-trinitrochlorobenzene (TNCB) in carrier. This response was specific for immunizing hapten. Furthermore, it was equally possible to induce ACD by intratracheal inoculation of trinitrophenyl- (TNP) modified bronchoalveolar cells (BAC) or haptenated spleen cells. Adoptive transfer studies demonstrated that the hypersensitivity that resulted from IT inoculation of TNBS or TNP BAC could be transferred with cells. With a monoclonal anti-hamster Ia reagent, it was demonstrated that, like Langerhans cells, 80 to 90% of cells in the bronchial lavage fluid expressed Ia determinants on their membrane. These cells were morphologically indistinguishable from macrophages. Because an individual is capable of being sensitized when hapten is introduced by the pulmonary or epicutaneous routes, the possibility is raised that the alveolar macrophages in the lung possess a similar capability for antigen presentation of hapten in the induction of ACD as does the Langerhans cell.