Prostaglandin D2-d4 (PGD2-d4) is intended for use as an internal standard for the quantification of PGD2 by GC- or LC-mass spectrometry. PGD2 is the major eicosanoid product of mast cells and is released in large quantities during allergic and asthmatic anaphylaxis. Mastocytosis patients produce excessive amounts of PGD2, which causes vasodilation, flushing, hypotension, and syncopal episodes. 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. Further pharmacological actions include inhibition of platelet aggregation and relaxation of vascular smooth muscle. PGD2 inhibits human ovarian tumor cell proliferation with an IC50 of 6.8 µM.