A reduced activity of BMAP18 and tobramycin due to DNA and mucin was detected. In addition, the peptide was observed to be degraded in the lung bronchoalveolar lavage in mice, explaining the scarce antimicrobial effect previously observed in vivo. For this reason a stereoisomeric form of the peptide, D-BMAP18, resistant to proteolytic degradation was prepared and tested. D-BMAP18 maintained its in vitro activity, and when used on a pulmonary acute infection healthy mice or with acute pulmonary infection by a P. aeruginosa strain the peptide showed to be almost active as tobramycin. The effectiveness of D-BMAP18 could be increased further whether the components of bronchoalveolar lavage that inhibit the activity of the peptide without degrading it will be identified. These results not only bring clinical applications of D-BMAP18 closer, but also draw a method of study which could be also applied to other AMPs.