The chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2) receptor, a G protein-coupled receptor that mediates chemotaxis of inflammatory cells in response to prostaglandin D 2 (PGD 2 ), is hypothesized to play a role in Th2-mediated allergic disease. In addition to PGD 2 , CRTH2 can be activated by indomethacin, a nonselective cyclooxygenase inhibitor and widely used nonsteroidal antiinflammatory drug (NSAID). To evaluate the structural features that confer CRTH2 binding selectivity, structure-activity relationship analysis of arylacetic acid class NSAIDs as CRTH2 receptor ligands was performed. Indomethacin, sulindac sulfide, and zomepirac displaced Prostaglandin D 2 (PGD 2 ), the predominant prostanoid produced by activated mast cells, is associated with allergic diseases such as allergic asthma and atopic dermatitis (Lewis et al., 1982;Murray et al., 1986;Barr et al., 1988). The biological effects of PGD 2 are mediated by two distinct G protein-coupled receptors: the D prostanoid receptor (DP) and the recently discovered chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2). Despite binding PGD 2 with high affinity, the CRTH2 receptor does not share significant sequence homology with the DP or other prostanoid receptors; instead, it exhibits greatest sequence similarity to chemoattractant receptors such as the formyl peptide receptor (Hirai et al., 2001). In humans, the CRTH2 receptor is expressed on Th2 cells and eosinophils (Nagata et al., 1999a,b) which are well known to play a role in the pathogenesis of allergic diseases such as allergic asthma (Hamid et al., 2003). In vitro activation of CRTH2 stimulates chemotaxis of human Th2 cells, eosinophils and basophils and in vivo activation leads to leukocyte mobilization in rats (Hirai et al., 2001;Shichijo et al., 2003), suggesting that CRTH2 may mediate recruitment of inflammatory cells in