Surfactant proteins A and D (SP-A, SP-D) can interact with lipopolysaccharide (LPS) and stimulate alveolar macrophages. The opsonic activities of SP-A and SP-D for bacteria with different types of LPS and alveolar macrophages were investigated. In flow cytometric studies with fluorescein-labeled rough (J5) and smooth (O111) Escherichia coli and rat alveolar macrophages, SP-A enhanced binding of J5 but not O111 bacteria to macrophages. Most importantly, SP-A enhanced ingestion of J5 bacteria by alveolar macrophages and subsequent bacterial killing. Immunoelectron microscopy demonstrated that J5 bacteria, the interface between the bacterium and the outer membrane of the alveolar macrophage, and ingested bacteria were heavily labeled with SP-A. In contrast, SP-D did not mediate phagocytosis. SP-A acted as an opsonin in the phagocytosis of rough LPS-containing bacteria by alveolar macrophages, emphasizing the possible role for SP-A in the alveolar defense system.