19(R)-Hydroxylated prostaglandins (PGs) are present at µg/ml concentrations in the semen of some mammalian species, notably primates, with the majority being from the PGE series and featuring a 15(S),19(R) hydroxyl stereochemistry. These compounds are also observed in marsupials' seminal plasma, where F-type 1 and 2-series compounds are predominant. The 15(R)-hydroxy epimer represents the inverse or "unnatural" isomer at C-15 for these 19-hydroxylated PGs. Although the biological function of 19(R)-hydroxylated PGs remains unclear, 19(R)-hydroxylation in the F-series leads to a notable reduction in receptor-mediated biological activity in certain assays.