The biological effects of prostaglandin D2 (PGD2) are transduced by at least two 7-transmembrane G protein-coupled receptors, designated DP1 and CRTH2/DP2. In humans, CRTH2/DP2 is expressed on Th2 cells, eosinophils, and basophils where it mediates the chemotactic activity of PGD2. CAY10597, as a racemic mixture, is a potent CRTH2/DP2 receptor antagonist that binds to the human receptor with a Ki value of 37 nM. The R enantiomer is slightly more potent exhibiting Ki values of 23 and 22 nM at the human and murine CRTH2/DP2 receptor, respectively. The R enantiomer of CAY10597 inhibits eosinophil chemotaxis induced by 13,14-dihydro-15-keto-prostaglandin D2 with an IC50 value of 40 nM.