The physiological actions of PGD2 include regulation of sleep, lowering of body temperature, inhibition of platelet aggregation and relaxation of vascular smooth muscle. PGD2 mediates its effects by 2 distinct G-protein-coupled receptors, DP1 and CRTH2/DP2. 15(R)-15-methyl PGD2 is a potent, selective agonist for the CRTH2/DP2 receptor. The EC50 values for eosinophil CD11b expression, actin polymerization, and chemotaxis are 1.4, 3.8, and 1.7 nM, respectively, each of which is approximately 3-5 fold lower than those for PGD2. In contrast the EC50 for the DP1-mediated increase in platelet cAMP by 15(R)-15-methyl PGD2 is >10 µM.