T o determine the role of surfactant protein-A(SP-A) in antiviral host defense, mice lacking SP-A (SP-A^–/–) were produced by targeted gene inactivation. SP-A^–/– and control mice (SP-A^+/+) were infected with respiratory syncytial virus (RSV) by intratracheal instillation. Pulmonary infiltration after infection was more severe in SP-A^–/– than in SP-A^+/+ mice and was associated with increased RSV plaque-forming units in lung homogenates. Pulmonary infiltration with polymorphonuclear leukocytes was greater in the SP-A^–/– mice. Levels of proinflammatory cytokines tumor necrosis factor-α and interleukin-6 were enhanced in lungs of SP-A^–/– mice. After RSV infection, superoxide and hydrogen peroxide generation was deficient in macrophages from SP-A^–/– mice, demonstrating a critical role of SP-A in oxidant production associated with RSV infection. Coadministration of RSV with exogenous SP-A reduced viral titers and inflammatory cells in the lung of SP-A^–/– mice. These findings demonstrate that SP-A plays an important host defense role against RSV in vivo.