To study the kinetics and function of alveolar macrophages after interruption of marrow function, we performed serial bronchoalveolar lavages in dogs. The studies were performed before and after 9.0 to 9.5 Grey total body irradiation and marrow infusion. Monocytes had disappeared from the bloodstream by Day 7 after the irradiation. Alveolar macrophages were significantly decreased at Day 21. At Days 14 and 21 myeloperoxidase-positive alveolar macrophages were also significantly decreased. Beyond Day 30 the number of circulating monocytes, myeloperoxidase-positive and total alveolar macrophages had returned. Sex chromatin stains of alveolar macrophages obtained from a male dog that received female marrow indicated that the repopulating macrophages were of marrow origin. In vitro studies of alveolar macrophage migration and phagocytosis demonstrated increased activities beyond Day 30. These studies suggest that in this model the alveolar macrophage is dependent on the bone marrow for support and that the alveolar macrophage depletion may impair lung defense mechanisms.