Antimicrobial peptides and proteins (APPs) are becoming increasingly important in targeting multidrug-resistant (MDR) bacteria. Inhalation delivery of these agents can be an effective treatment of respiratory infections owing to the high local drug concentration in the lungs. Endolysins are bacteriophage-derived protein molecules highly effective for bacterial killing. Cpl- 1 and ClyJ-3 are native and chimeric endolysins, respectively, having antimicrobial activity against Streptococcus pneumoniae which causes lung infections. In chapter 2, we conducted the first feasibility study on nebulisation of Cpl-1 and ClyJ-3, with a focus on the antimicrobial activity, structural changes of the proteins and aerosol performance. Nebulisation caused almost complete losses in antimicrobial effect and bioactivity of ClyJ-3. In contrast, the bactericidal activity of Cpl-1 showed no significant difference (p≥0.05) before and after mesh nebulisation. However, jet nebulisation reduced the bioactivity of Cpl-1, and the effect was time-dependent. Chapter 3 shows the first study to investigate the feasibility of spray-dried endolysins Cpl-1 and ClyJ-3 with excipients to produce inhalable powders. The two endolysins were individually tested with leucine and sugar (lactose or trehalose) for spray drying method. A complete loss of ClyJ-3 bioactivity was observed after atomization of the liquid feed solution (before the drying process), while Cpl-1 maintained its bioactivity in the spray-dried powders with promising physico-chemical properties and aerosol performances. Chapter 4 indicates that in a murine S. pneumoniae lung infection model, single treatment of Cpl-1 in liquid or powder decreased the pulmonary bacterial load by approximately 1 log, while the Cpl-1 and gentamycin combination treatment caused a 2.5 log reduction of bacterial counts. The results indicated that inhalation formulations of endolysin hold the potential for respiratory infection treatment.