We investigated the role of prostaglandin D 2 (PGD 2 ) signaling in acute lung injury (ALI), focusing on its producer–effector interaction in vivo. Administration of endotoxin increased edema and neutrophil infiltration in the WT mouse lung. Gene disruption of hematopoietic PGD synthase (H-PGDS) aggravated all of the symptoms. Experiments involving bone marrow transplantation between WT and H-PGDS–deficient mice showed that PGD 2 derived from alveolar nonhematopoietic lineage cells (i.e., endothelial cells and epithelial cells) promotes vascular barrier function during the early phase (day 1), whereas neutrophil-derived PGD 2 attenuates its own infiltration and cytokine expression during the later phase (day 3) of ALI. Treatment with either an agonist to the PGD 2 receptor, DP, or a degradation product of PGD 2 , 15-deoxy-Δ 12,14 -PGJ 2 , exerted a therapeutic action against ALI. Data obtained from bone marrow transplantation between WT and DP-deficient mice suggest that the DP signal in alveolar endothelial cells is crucial for the anti-inflammatory reactions of PGD 2 . In vitro, DP agonism directly enhanced endothelial barrier formation, and 15-deoxy-Δ 12,14 -PGJ 2 attenuated both neutrophil migration and cytokine expression. These observations indicate that the PGD 2 signaling between alveolar endothelial/epithelial cells and infiltrating neutrophils provides anti-inflammatory effects in ALI, and suggest the therapeutic potential of these signaling enhancements.