The trans isomer of PGD2 occurs as an impurity between 2-5% in most commercial preparations of the bulk drug product. This compound was prepared primarily as an analytical standard for detection and quantitation of this impurity. From what can be inferred from the study of other trans isomers of F-type prostaglandins, the biological activity of 5-trans PGD2 is likely to be similar to that of the cis isomer. However, there are no specific published reports on the biological activity of 5-trans PGD2.