15(R)-PGE2 is the C-15 epimer of the naturally occurring 15(S)-PGE2. 15(R)-PGE2 is much less potent than the natural stereoisomer in most biological assays. In hamsters, a dose of 0.20 mg of PGE2 inhibits fertility by 100%, whereas a dose of 1.0 mg of 15(R)-PGE2 inhibits fertility by only 17%. 15(R)-PGE2 occurs as a minor impurity in some pharmaceutical formulations of PGE2 used in obstetrics.