OBJECTIVE: Lysozyme, a 14-kDa protein, is one of the most abundant antimicrobials in the lungs. Its concentration in airway surface sufficient to kill several bacterial pathogens in vitro. The purpose of this study was to determine if administration of exogenous lysozyme would further enhance bacterial killing in vivo. METHODS: To assess the effect of acute lung infection on endogenous lysozyme protein levels, mice were infected by intratracheal instillation of Pseudomonas aeruginosa and bronchoalveolar (BAL) fluid assessed for lysozyme concentration and for muramidase activity. In order to inform in vivo testing, species-specific bacterial killing efficacy was determined by incubating mucoid P. aeruginosa with 2×10 units of chicken lysozyme, human lysozyme or with vehicle at 37°C for 2hours. Subsequently, mice challenged with intratracheally-administered mucoid P. aeruginosa, were reintubated and injected with 2×10 Units of native human lysozyme, recombinant human lysozyme or with vehicle. Lung bacterial burden was enumerated subsequently. RESULTS: The concentration of lysozyme protein in BAL fluid from mice challenged with mucoid clinical isolate of P. aeruginosa was increased 4-fold at 6hours post-infection. Quantitative culture showed that the number of recoverable bacteria was significantly decreased by both chicken and human lysozyme compared to vehicle but human lysozyme was significantly more effective than chicken egg lysozyme. In vivo, 24hours post-infection quantitative culture of lung homogenates showed that the number of viable bacteria recovered from mice treated with either native or recombinant lysozyme was decreased with 0.76±0.25×10 and 0.84±0.16×10, respectively, vs. 7.0±2.52×10 CFU/g protein in mice treated with HBSS, both P<0.05. CONCLUSIONS: These results indicate that endogenous lysozyme is increased during acute lung infection and that early administration of exogenous lysozyme further enhances bacterial killing in vivo.