To determine the roles of the type 1 tumor necrosis factor (TNF) receptor (TNFR1) in lung inflammation and antibacterial defense, we exposed transgenic mice lacking TNFR1 [TNFR1(−/−)] and wild-type control mice to aerosolized lipopolysaccharide orPseudomonas aeruginosa. After LPS, bronchoalveolar lavage fluid (BALF) from TNFR1(−/−) mice contained fewer neutrophils and less macrophage inflammatory protein-2 than BALF from control mice. TNF-α, interleukin-1β, and total protein levels in BALF as well as tissue intercellular adhesion molecule-1 expression did not differ between the two groups. In contrast, lung inflammation and bacterial clearance after infection were augmented in TNFR1(−/−) mice. BALF from infected TNFR1(−/−) mice contained more neutrophils and TNF-α and less interleukin-1β and macrophage inflammatory protein-2 than that from control mice, but protein levels were similarly elevated in both groups. Lung inflammation and bacterial clearance were also augmented in mice lacking both TNF receptors. Thus TNFR1 facilitates neutrophil recruitment after inhalation of lipopolysaccharide, in part by augmenting chemokine induction. In contrast, TNFR1 attenuates lung inflammation in response to live bacteria but does not contribute to increased lung permeability and is not required for the elimination ofP. aeruginosa.