Objectives: We describe the antimicrobial activity against Pseudomonas aeruginosa of the de novo-derived antimicrobial peptide WLBU2 in an animal model of infection. Methods: For this study, an intravenous (iv)model of P. aeruginosa infection was established. The minimum lethal murine dose of P. aeruginosa strain PA01 was determined to be 3 × 107 cfu when bacteria were administered iv. Increasing concentrations of WLBU2 were instilled either prior to or following PA01 septic exposure. Results: For the mice given peptide post-bacterial infection, in the 1 mg/kg group, nine of nine animals died because of Pseudomonas sepsis; in the 3 mg/kg group, only one of nine succumbed to infection and in the 4 mg/kg group, all mice were protected (P < 0.0001). Similar results were obtained when WLBU2 was given 1 h prior to Pseudomonas infection. Conclusions: Although the therapeutic window in this model is narrow, the results nonetheless provide encouraging evidence for WLBU2 as a potential prophylactic or treatment of bacterial infection. © The Author 2007. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved.