Thus, the in vivo roles of endogenous cathelicidin in host defence against P. aeruginosa, the relative effects of microbicidal and modulatory properties, and the consequences of therapeutic targeting of cathelicidin expression or exogenous delivery of peptide remain unknown. We demonstrate that therapeutically administered synthetic LL37 peptide can enhance the clearance of P. aeruginosa from the murine lung, in the absence of demonstrable direct microbicidal effects, and can induce an upregulation of the early neutrophil response to pathogen in the lungs that is dependent both upon the presence of the peptide and the pathogen. We show that despite a normal early neutrophil response, second phase pulmonary neutrophil influx was deficient in Camp2/2 mice, with impaired clearance of pulmonary P. aeruginosa. Delivery of LL-37 to these cathelicidin-deficient mice enhanced the neutrophil response and restore bacterial clearance, demonstrating proof of principle for therapeutic use of LL-37 in cathelicidin deficiency. These studies indicate that the protective effects of cathelicidins in P. aeruginosa infection in vivo can result from modulatory effects in innate immune responses, synergising with infectious stimuli to enhance a protective neutrophil response. Cathelicidins are recognised as key multifunctional modulators of innate immunity and host defence against infection, and offer possible novel therapeutic templates. In addition to directly microbicidal potential, these peptides have been described as having a broad range of inflammomodulatory and immunomod ulatory properties. However, no clear evidence exists for these functions being involved in cathelicidin-mediated enhanced host defence against pulmonary infection in vivo, with the relative significance of microbicidal potential and modulatory functions remaining unclear. Using a murine model of acute P. aeruginosa lung infection, we demonstrate cathelicidin-mediated enhancement of bacterial clearance in vivo in the absence of direct early microbicidal activity. Administration of synthetic LL-37 promoted an upregulation of the early neutrophil response that was dependent upon both infection and peptide, but was independent of native cathelicidin production, and enhanced bacterial clearance from the lung. The associations between hCAP-18/LL-37 expression and susceptibility to infection in humans suggest an important, but as yet undefined role for hCAP-18/LL-37 in innate host defence against infection in humans. Although this peptide has microbicidal potential, its activity is poor against many microorganisms in physiologically relevant environments at the low concentrations found in vivo in most systems. However, cathelicidins have additionally been shown to have multiple modulatory activities, including chemotactic function, the ability to modulate chemokine, cytokine and cellular responses.