17-phenyl trinor PGD2 is a novel, chemically stable analog of PGD2 wherein the lower side chain is modified by the addition of a phenyl group at C-17 in place of the last 3 ω-chain carbon atoms. This modification has not been reported in a D-type prostaglandin. However, in the PGE and PGF series, the analogous modification leads to a stable, somewhat more potent agonist which embodies the same biological activities as the parent prostaglandins.