PGE2 p-acetamidophenyl ester is a crystalline derivative of PGE2. PGE2 is one of the primary COX products of arachidonic acid and one of the most widely investigated PGs. Its activity influences inflammation, fertility and parturition, gastric mucosal integrity, and immune modulation.{1585,1558,437,1330} The effects of PGE2 are transduced by at least four distinct receptors designated EP1, EP2, EP3, and EP4.{1422} Affinity constants (Kd) of PGE2 for these receptors range from 1-10 nM depending on the receptor subtype and tissue.