A canine model for the comparative study of lower respiratory and systemic immunologic reactions is described. The techniques of bronchopulmonary lavage and percutaneous splenic aspiration were employed to obtain serial samples of pulmonary and systemic lymphocytes. The morphologic features and the yield of cells obtained by these procedures were characterized. The presence of lymphocytes and plasma cells among the cell populations obtained by pulmonary lavage was confirmed by electron microscopy. The effect of serial bronchopulmonary lavage per se on the yield, composition, and functional properties of bronchoalveolar lymphocytes and macrophages was found to be minor. By contrast, the effect of immununization on these parameters was marked and easily detected. Evidence is presented that at least 2 anatomically and functionally different populatons of immunocytes subserve the lower respiratory tract, namely, the hilar lymph node cells and the bronchoalveolar lymphocytes. It is concluded that the canine model is well suited for the investigation of respiratory and systemic immunity and is useful for the study of bronchoalveolar lymphocyte and alveolar macrophage function, either independently or in concert with the study of pulmonary host defense and the pathogenesis of lung disease.