ACTIMMUNE in Intermediate Osteopetrosis

Inclusion Criteria: * Diagnosis of osteopetrosis; and * Anemia (Hemoglobin \<12 g/dL) not related to iron deficiency, or * Neutropenia (Neutrophil count \<1000 neutrophils/ul unsupported with cytokines), or * Thrombocytopenia (Platelet count \<50,000 cells x 109/L), or * History of impaired bone healing, or * ≥ 1 serious infection over prior year defined as requiring hospitalization and/or IV antibiotics, and * Age \> 1 year; and * Ability to travel to a study center for every 3-6 month study visits; and * Patient or parent/legal guardian is able and willing to provide informed consent. For patients 7 to 17 years of age, assent must also be provided. Exclusion Criteria: * 12 months or fewer following HCT; * Pregnancy or breastfeeding; * Known or suspected allergy to interferon gamma-1b or related products; * Participation in simultaneous therapeutic study that involves an investigational study drug or agent within 4 weeks of study enrollment; * ALT greater than 3 fold higher than normal; or * Any other social or medical condition that the Investigator believes would pose a significant hazard to the subject if the investigational therapy were initiated or be detrimental to the study.