Summary The ability of several Pseudomonas aeruginosa exo-enzymes, including exotoxin A (ETA), to induce inflammation and their influence on endotoxin-induced tumour necrosis factor (TNF) production in murine lung were evaluated. Intratracheal administration of lipopolysaccharide (LPS; 0.1-10 μg/mouse), 2-1 LD50 of P. aeruginosa alkaline protease (7.5 μg/mouse) and elastase (1.2 μg/mouse) elevated total cell number and the percentage of neutrophils in broncho-alveolar lavage fluid (BALF), whereas ETA (0.1 μg/mouse) did not. LPS induced TNF production in BALF in a dose-dependent manner, whereas the P. aeruginosa exo-enzymes did not. When ETA was inoculated into the respiratory tract before LPS, production of TNF in BALF was significantly suppressed in a dose-dependent manner. ETA also suppressed TNF production by alveolar macrophages (AMs) stimulated with LPS in vitro. Flow cytometric analysis showed that ETA markedly reduced the expression of CD14 and CD11c/CD18 on the surface of AMs. ETA also depressed partially the expression of TNF-α mRNA in AMs. These findings suggest that ETA regulates TNF production in murine lung by suppressing LPS receptor expression, mRNA expression and protein synthesis and/or secretion of TNF.