The purpose of this research study is to evaluate the safety, tolerability, and efficacy of Interferon gamma-1b (IFN-gamma 1b) when administered by subcutaneous injection over a period of 4 weeks to patients with mild-to-moderate cystic fibrosis. Additionally, preliminary assessments on the effects of IFN-gamma 1b on lung function and other indicators of health will be made.

• At least 6 years of age
• Diagnosis of cystic fibrosis (against certain criteria)
• Able to perform pulmonary (lung) function tests and participate in induced sputum procedures
• Pulmonary function values must meet certain minimal requirements
• Must have acceptable laboratory test results
• Cannot be on certain medications during and immediately prior to study
• Cannot have a history of unstable or deteriorating cardiac or neurologic disease