Iloprost is a second generation structural analog of prostacyclin (PGI2) with about ten-fold greater potency than the first generation stable analogs, typified by carbaprostacyclin.{3347} Iloprost binds with equal affinity to the human recombinant IP and EP receptors with a Ki of 11 nM.{8322} 15(R)-Iloprost is the ``unnatural'' or inverted C-15 epimer of iloprost. This transformation frequently attenuates the biological agonist activity of prostaglandin analogs by several orders of magnitude. There are no literature reports of the biological activity of 15(R)-iloprost.