Prostaglandin D2 (PGD2) is biosynthesized in the brain by a soluble, 26 kDa glutathione-independent lipocalin-type PGD2 synthase. This PGD2 accumulates in the cerebrospinal fluid (CSF), where it induces physiologic sleep in rats and humans. PGD2 is also synthesized in mast cells and leukocytes by a cellular, myeloid-type, glutathione-dependent PGD synthase. This PGD2 which is formed in the intracellular and vascular compartments is rapidly metabolized to 11β-PGF2α and other metabolites. Measurement of the parent eicosanoid PGD2 is appropriate in cell culture lysates and in CSF, where concentrations of several hundred pg/ml have been measured. Cayman’s PGD2 ELISA Kit is a competitive assay that can be used for quantification of PGD2 in cell culture lysates and enzymatic reactions.