Hypersensitivity pneumonitis (HP) is an interstitial lung disease that develops following repeated exposure to inhaled particulate antigens. The disease is characterized by lymphocytic alveolitis, granuloma formation and fibrosis. IFN-gamma is required for the formation of granulomas in HP, and we therefore focused on identifying the cellular sources of IFN-gamma during the disease. Using the Saccharopolyspora rectivirgula (SR) animal model of HP, we demonstrated that the majority of IFN-gamma(+) cells in the lung following SR exposure are neutrophils. Ab-mediated depletion of neutrophils in mice prior to exposure to SR resulted in a decrease in the level of IFN-gamma mRNA and protein compared to isotype Ab-treated mice, suggesting that neutrophils are an important source of IFN-gamma during HP. To determine the contribution of T and non-T cell sources of IFN-gamma to granuloma formation, we performed adoptive transfer studies. RAG-1(-/-) mice reconstituted with spleen cells from IFN-gamma(-/-) mice developed granulomas similarly to RAG-1(-/-) mice reconstituted with normal spleen cells. Therefore innate immune cell IFN-gamma production in the absence of T cell IFN-gamma production is sufficient for granuloma formation. These results provide new insight into the pathogenesis of HP and demonstrate the important contribution of innate immune cells to the disease process.