Prostaglandin D2-d9 (PGD2-d9) is intended for use as an internal standard for the quantification of PGD2 by GC- or LC-MS. PGD2 is the major eicosanoid product of mast cells and is released in large quantities during allergic and asthmatic anaphylaxis.¹ Mastocytosis patients produce excessive amounts of PGD2, which causes vasodilation, flushing, hypotension, and syncopal episodes.¹ PGD2 is also produced in the brain via an alternative pathway involving a soluble, secreted PGD-synthase also known as β-trace.^2,3 In the brain, PGD2 produces normal physiological sleep and lowering of body temperature.^2,3 Further pharmacological actions include inhibition of platelet aggregation and relaxation of vascular smooth muscle.⁴ PGD2 inhibits human ovarian tumor cell proliferation with an IC50 value of 6.8 µM.⁵