The purpose of this research study is to evaluate the safety and effectiveness of Interferon gamma-1b (IFN-g 1b) on lung function when given to patients with cystic fibrosis by inhalation (breathed into the lungs) three times a week for 12 weeks.

The FDA has not approved Interferon gamma-1b for use with cystic fibrosis patients, which is the condition being examined in this study.

Key inclusion criteria:

• At least 12 years of age
• Diagnosis of cystic fibrosis with mild to moderate pulmonary impairment
• Must be receiving ongoing chronic treatment with TOBI (inhaled tobramycin) OR not receiving ongoing chronic treatment with TOBI and no use of TOBI or other inhaled antibiotic within 4 weeks prior to study drug administration·
• Other specific diagnostic indicators of CF and other factors must meet minimum requirements.