The role of surfactant-associated protein (SP) A in the mediation of pulmonary responses to bacterial lipopolysaccharide (LPS) was assessed in vivo with SP-A gene-targeted [SP-deficient; SP-A(−/−)] and wild-type [SP-A(+/+)] mice. Concentrations of tumor necrosis factor (TNF)-α, macrophage inflammatory protein-2, and nitric oxide were determined in recovered bronchoalveolar lavage fluid after intratracheal administration of LPS. SP-A(−/−) mice produced significantly more TNF-α and nitric oxide than SP-A(+/+) mice after LPS treatment. Intratracheal administration of human SP-A (1 mg/kg) to SP-A(−/−) mice restored regulation of TNF-α, macrophage inflammatory protein-2, and nitric oxide production to that of SP-A(+/+) mice. Other markers of lung injury including bronchoalveolar fluid protein, phospholipid content, and neutrophil numbers were not influenced by SP-A. Data from experiments designed to test possible mechanisms of SP-A-mediated suppression suggest that neither binding of LPS by SP-A nor enhanced LPS clearance are the primary means of inhibition. Our data and others suggest that SP-A acts directly on immune cells to suppress LPS-induced inflammation. These results demonstrate that endogenous or exogenous SP-A inhibits pulmonary LPS-induced cytokine and nitric oxide production in vivo.