Mast cell-derived prostaglandin D 2 (PGD 2 ) is one of the essential modulators of eosinophilic airway inflammation in asthma and allergic rhinitis. Two G protein-coupled receptors for PGD 2 , prostaglandin D 2 receptor (DP) and chemoattractant receptorhomologous molecule expressed on Th 2 cells (CRTH2), are both expressed on the surface of eosinophils, and CRTH2 has been demonstrated to mediate PGD 2 -induced eosinophil mobilization in vitro. However, it has not yet been determined whether PGD 2 and its receptors mediate in vivo eosinophil trafficking into the airways or other organs. We demonstrated that intratracheal administration of PGD 2 in rats pretreated with systemic interleukin-5 (IL-5) injection induced marked airway eosinophilia, determined by the differential counts of cells in bronchoalveolar lavage (BAL) fluid and lung histology, within 2 h. Systemic IL-5 alone significantly increased the number of eosinophils in the peripheral blood but showed no effect on airway eosinophilia. Three CRTH2-specific agonists (13,14-dihydro-15-keto-PGD 2 , 11-deoxy-11-methylene-15-keto-PGD 2 , and indomethacin) demonstrated equivalent induction of BAL eosinophilia to that of PGD 2 , but a DP agonist (BW 245C [5-(6-carboxyhexyl)-1-(3-cyclohexyl-3-hydroxypropyl)-hydantoin]) or a thromboxane A 2 receptor (TP) agonist ([1S-1␣,2(5Z), 3␣(1E,3R*),4␣)]-7-[3-(3-hydroxy-4-(4Ј-iodophenoxy)-1-butenyl)-7-oxabicyclo-[2.2.1]heptan-2-yl]-5-heptenoic acid) showed no effect. PGD 2 or CRTH2 agonist-induced BAL eosinophilia was almost completely inhibited by pretreatment with a CRTH2/TP antagonist, ramatroban [BAY-u3405; (ϩ)-(3R)-3-(4-fluorobenzenesulfonamido)-1,2,3,4-tetra-hydrocarbazole-9-propionic acid], whereas a TP-specific antagonist, SQ29,, or a DP-specific antagonist, BW A868C [3-benzyl-5-(6-carboxyhexyl)-1-(2-cyclohexy-2-hydroxyethylamino)-hydantoin], did not inhibit the effects of PGD 2 . These results suggest that CRTH2 plays a significant role in the eosinophil trafficking from the bloodstream into the airways in PGD 2 -related airway inflammation.