Pulmonary surfactant isolated from bronchoalveolar lavage fluid of rat lung contained a high content of surfactant protein A (SP-A) in starved for 2 days compared to fed controls, but this phenomena returned to baseline following more than 4 days starvation. As determined by immunoperoxidase staining of lung sections using SP-A antibody, SP-A could be consistently observed in nonciliated bronchiolar (Clara) cells, alveolar type II cells and some alveolar macrophages (AM). Fc receptor-mediated phagocytosis of AM was enhanced by SP-A, which was dependent on the dosis and reached a maximum at 10 micrograms of SP-A/ml. Antibody to SP-A completely inhibited the enhanced response of phagocytosis. When exposed AM subpopulations, separated into four fractions (I, II, III and IV) by discontinuous Percoll gradient, to SP-A or pulmonary surfactant prepared from rats fed and starved for 2 days enhanced their phagocytic activity in high dense cells (III and IV), particularly to SP-A and pulmonary surfactant from rats starved for 2 days. Whereas little change in lower dense fractions (I and II) were seen in all exposures except for SP-A that enhanced the cells of fraction II. These results supported the concept that pulmonary surfactant and its apoprotein, SP-A, are a factor to regulate lung defense system including activation of AM that undergo different processes following starvation.