The mechanisms by which T cells accumulate in the lungs of patients with pulmonary fibrosis are poorly understood. Because the lung is continually exposed to microbial agents from the environment, we repeatedly exposed C57BL/6 mice to the ubiquitous microorganism, Bacillus subtilis, to determine whether chronic exposure to an inhaled microorganism could lead to T cell accumulation in the lungs and subsequent pulmonary fibrosis. C57BL/6 mice repeatedly treated with B. subtilis for 4 consecutive weeks developed a 33-fold increase in the number of CD4(+) T cells and a 354-fold increase in gamma delta T cells in the lung. The gamma delta T cells consisted almost entirely of V gamma 6/V delta 1(+) cells, a murine subset bearing an invariant TCR the function of which is still unknown. Treatment of C57BL/6 mice with heat-killed vs live B. subtilis resulted in a 2-fold increase in the number of CD4(+) T cells in the lung but no expansion of gamma delta T cells indicating that gamma delta cells accumulate in response to live microorganisms. In addition, mice treated with heat-killed B. subtilis developed significantly increased pulmonary fibrosis compared with mice treated with the live microorganism. Mice deficient in V gamma 6/V delta 1(+) T cells when treated with B. subtilis had a 231-fold increase in lung CD4(+) T cells and significantly increased collagen deposition compared with wild-type C57BL/6 mice, consistent with an immunoregulatory role for the V gamma 6/V delta 1 T cell subset. These findings indicate that chronic inhalation of B. subtilis can result in T cell accumulation in the lung and fibrosis, constituting a new model of immune-mediated pulmonary fibrosis..