The purpose of this research study is to test the safety and effectiveness of inhaled Interferon gamma-1b (IFN-g 1b), when administered for 48 weeks and in combination with oral antibiotics which may be administered for up to 72 weeks for the treatment of a lung infection caused by a bacterium called Mycobacterium avium complex .

FDA has not approved Interferon gamma-1b for use in patients with MAC infection of the lungs, which is the purpose of this study.Interferon gamma-1b and similar proteins play important roles in establishing and maintaining protective immune responses against a variety of microorganisms.

Laboratory research has shown that a mouse form of interferon gamma is effective in treating mice with infections caused by bacteria similar to MAC as well as MAC infection.

Male and female patients, 18 years of age or older, with bacteriologically and radiographically confirmed pulmonary MAC infection who have been treated for their infection for at least 6 months within the previous 2 years, as well as patients with moderate or severe pulmonary disease due to MAC, not previously treated for this infection. Patients with recurrent pulmonary MAC infection after previous successful treatment for pulmonary MAC disease are also eligible.

In addition, various laboratory testing must confirm conditions. Patients cannot be positive for HIV or have an extra-pulmonary (in general, outside of the lungs) infection of MAC. Other preconditions related to health or other conditions exist as factors for inclusion or exclusion from this study.