The present study was performed to characterize the immunologic potential of interstitial macrophages (INT) in comparison with alveolar macrophages (AL). The data showed that AL, compared with INT, have a more efficient phagocytic potential. In addition, they have a strong microbicidal activity and secrete large amounts of reactive oxygen radicals, nitric oxides, TNF, and IFN on appropriate stimulation. They also exert strong tumoricidal and parasiticidal activities. In contrast, INT are more efficient in releasing immunoregulatory cytokines such as IL-1 and IL-6. As determined by Ab staining, INT express more MHC class II molecules and are more effective in functioning as accessory cells for mitogen-stimulated lymphocyte proliferation compared with AL. Thus, AL appear to be particularly effective as nonspecific first line defense cells against infectious agents, whereas INT are equipped to cooperate with interstitial lymphocytes in inducing a specific immune reaction.