ABSTRACT The antibacterial activities of the cathelicidin peptides LL-37 and an 18-residue C-terminal fragment of prophenin, corresponding to positions 62 to 79 of native prophenin (PF-18), were analyzed in the presence of a modified surfactant preparation isolated from minced porcine lungs. At low micromolar concentrations, both LL-37 and PF-18 showed significant activities against different serotypes of group B streptococci, with LL-37 being more active on a molar basis. The surfactant preparation at a concentration of 10 mg/ml partly blocked the antibacterial activity of 9 μM LL-37 and completely blocked the antibacterial activity of 9 μM PF-18. However, 10 mg of the surfactant preparation per ml had only minor inhibitory effects on LL-37 and PF-18 at 90 μM. Addition of up to 900 μM PF-18 did not affect the surface properties of the surfactant preparation. These data suggest that surfactant preparations containing antimicrobial peptides could be useful for the local treatment of pulmonary infections.