The expression of inflammatory mediators by various cells following in vitro CD40 ligation is well known. However, knowledge of the role and interaction with these cells in the establishment and maintenance of in vivo immune-mediated inflammation is limited. In this report, a chimeric mouse model based on CD40 knockout and wild-type mice was used to assess the role of bone marrow (BM)-derived and non-BM-derived cells in a CD40-mediated pulmonary inflammation response. CD40+ BM-derived cells were required for initial cell recruitment, pulmonary edema, and weight loss associated with this response. The structural CD40+ non-BM-derived cells of the lung, such as fibroblasts, epithelial cells, and endothelial cells, could not by themselves establish any level of pulmonary inflammation. However, both the CD40+ BM-derived cells and the structural CD40+ non-BM-derived cells of the lung were required to maximize the level of pulmonary inflammation. Both B cells and T cells played a contributing role in macrophage recruitment and pulmonary edema but neither contributed to the inflammation-associated weight loss. These experiments indicate that CD40+ BM-derived cells were critical to the induction of pulmonary inflammation and that alveolar macrophages, B cells, and T cells contributed to selective aspects of the response.