We recently described overproduction of interferon (IFN)- γ by lung lymphocytes in mice with silicosis (11% of cells in air-control versus 19% of cells from silica-exposed mice; Davis and colleagues, Am. J. Respir. Cell Mol. Biol. 1999;20:813–824). We hypothesized that the increased IFN- γ production might be due to selective enrichment of one lymphocyte phenotype. To test this hypothesis, small mononuclear cells from lung digest preparations of mice exposed 4 mo previously to cristobalite silica (70 mg/m³, 12 d, 5 h/d) or to sham-air were stained for intracellular cytokines and surface antigen phenotypes, and examined by flow cytometry. Air-sham mouse lung digests included CD4⁺ (16%) and CD8⁺ (6%) T cells, γδ T-cell antigen receptor (TCR)⁺ CD4⁻CD8⁻ T cells (3%), natural killer (NK) cells (15%), B cells (6%), and macrophages (12%). The total number of lung lymphocytes was increased 1.7-fold in silicosis, but the phenotype frequencies did not change significantly. In the control lungs IFN- γ was produced by three major phenotypes of lymphocytes: 5% of CD4⁺ T cells, 5% of γδ -TCR⁺ CD4⁻CD8⁻ T cells, and 2% of NK cells. The percentage of each type producing IFN- γ was increased 2- to 3-fold in silicosis. When multiplied by cell number, the increased percentages yielded a 3- to 5-fold increase in the total number of each IFN- γ –producing phenotype in the lung. Our results demonstrate no selective phenotype enrichment but upregulated IFN- γ production by at least three lymphocyte phenotypes. IFN- γ may be an important signal driving lymphocyte differentiation and macrophage activation in silicosis.