Patients with cystic fibrosis (CF) suffer from chronic infections of the lower respiratory tract that can be caused by one or multiple bacteria, including Pseudomonas aeruginosa, which has been particularly problematic to eradicate and been implicated as the major cause of morbidity and mortality in CF patients. Aerosol delivery of antibiotics directly to the lung increases the local concentrations of antibiotic at the site of infection resulting in improved antimicrobial effects compared to systemic administration. Bacterial resistance to current aerosol antibiotic treatments indicate a need for improved therapies to treat CF patients with pulmonary infections caused by multi-drug resistant Pseudomonas aeruginosa and other bacteria. High concentrations of MP-376 delivered directly to the lung are projected to have antimicrobial effects on even the most resistant organisms. MP-376 is a novel formulation of the fluoroquinolone levofloxacin that has been optimized for aerosol delivery using the PARI electronic eFlow® nebulizer. Preclinical and early clinical studies in adults show that aerosol doses of MP-376 appear to be safe and well tolerated, and exert an antimicrobial effect when administered once or twice daily. High concentrations of levofloxacin in the lung delivered using MP-376 are expected to be active against CF pathogens such as P. aeruginosa and S. aureus, including those resistant to aminoglycosides (such as TOBI®) and other inhaled antimicrobial agents. Inhaled MP-376 can be delivered rapidly and efficiently using the PARI eFlow® nebulizer system. This Phase 1 study is being performed to obtain safety, tolerability and PK data in children ages 6-16 in order to determine if MP-376 is safe, prior to enrolling children of these ages in the planned pivotal Phase 3 studies.