Repeated intratracheal (IT) inoculation of rabbits with a homogenized, saline suspension of Micropolyspora faeni produced bronchopulmonary (BP) histologic lesions resembling those of human hypersensitivity pneumonitis. With an in vitro phagocytic and bactericidal assay, an analysis of BP macrophages from M. faeni-injected rabbits demonstrated activation at both 2 and 4 weeks after the initiation of immunization. No BP macrophage activation was observed in immunized rabbits 6 weeks post-inoculation. BP macrophage activation was capable of recall after 6 weeks in M. faeni-sensitized animals that received a booster IT injection (2 mg) that did not activate “normal” alveolar wash cells. This recall of BP macrophage activation was accompanied by both a marked migration of mononuclear cells into the lung and positive delayed hypersensitivity skin reactions after intradermal injection of M. faeni antigen. Pulmonary histologic examination of sensitized, boosted rabbits suggested an enhanced cellular parenchymal infiltrate when compared with appropriate controls. The above observations confirm the occurrence of immunologically activated BP macrophages in rabbits inoculated with M. faeni via the respiratory tract route and suggest a correlation between macrophage activation and histopathology.