19(R)-Hydroxylated PGs occur in µg/ml concentrations in the semen of certain mammalian species, especially primates. In the case of humans, the compounds are primarily of the PGE series, and the hydroxyl stereochemistry is 15(S),19(R). 19(R)-Hydroxylated PGs are also found in the seminal plasma of marsupials, where F-type compounds of the 1 and 2-series predominate. The 15(R)-hydroxy epimer of these 19-hydroxylated PGs is the inverse or “unnatural” isomer at C-15. The biological role of 19(R)-hydroxylated PGs is not well established. In the F-series, 19(R)-hydroxylation is associated with a significant loss of receptor-mediated biological activity in some assays.