Prostaglandin D2 (PGD2) is one of the five primary enzymatic prostaglandins derived directly from PGH2. PGD2 is produced abundantly in the CSF by the lipocalin-type PGD synthase, and in the periphery by myeloid cells including mast cells and basophils by a second, hematopoietic-type PGD synthase. PGD2 is chemically unstable, and its use and analysis is complicated by its short in vivo half-life. Δ12-PGD2 is one of the initial chemical decomposition products of PGD2. Δ12-PGD2 is an intermediate in the pathway leading to Δ12-PGJ2, which is a cyclopentenone prostaglandin with antimitotic and carcinogenic activities.^1,2 The metabolism of Δ12-PGD2 involves addition of thiol nucleophiles, as is the case with the majority of cyclopentenone prostaglandins.³