Tumor necrosis factor alpha (TNF) has been shown to be an essential cytokine mediator of innate immunity in Klebsiellapneumonia. Recently, a TNF agonist peptide consisting of the 11-amino-acid TNF binding site (TNF_70-80) has been shown to possess many of the leukocyte-activating properties of TNF without the associated toxicity when administered locally or systemically. Given the beneficial effects of TNF in gram-negative pneumonia, we hypothesize that the intratracheal (i.t.) administration of TNF_70-80 would augment lung innate immunity in mice challenged with intrapulmonary Klebsiella pneumoniae. The administration of TNF_70-80 i.t. to CBA/J mice 7 days prior to, but not concomitantly with, the i.t. delivery of 3 × 10³ CFU of K. pneumoniae resulted in a marked increase in survival compared to that of animals receiving a control peptide i.t. In addition, pretreatment with TNF_70-80resulted in improved bacterial clearance, which occurred in association with enhanced lung myeloperoxidase activity (as a measure of lung polymorphonuclear leukocyte influx), and increased expression of the important activating cytokines TNF, macrophage inflammatory protein-2, interleukin-12, and gamma interferon compared that for animals receiving control peptide. Finally, the administration of TNF_70-80 intraperitoneally resulted in enhanced rather than decreased lethality of Klebsiella pneumonia compared to that for animals receiving either TNF_70-80 or control peptide i.t. Our studies suggest that the intrapulmonary, but not systemic, administration of the TNF agonist peptide may serve as an important immunoadjuvant in the treatment of murineKlebsiella pneumonia.