Phagocytic activity of alveolar macrophages (AM) was enhanced by pulmonary surfactant obtained from bronchoalveolar lavage fluid of rats starved for 2 days, as compared to fed. The enhanced activity of phagocytosis was dependent on the dose of surfactant. The prepared surfactant showed a different protein to phospholipid ratio of 0.108 in fed and 0.234 in 2 days starved, because of an increased ratio of protein in surfactant from 2 days starved rats. F(ab')2 anti-surfactant protein inhibited the enhanced AM phagocytosis by surfactant. These results suggested that the enhancement of AM phagocytosis in 2 days starved rats was on account of an increase of protein in their surfactant compared to fed.