Prostaglandin D2 (PGD2) is biosynthesized in the brain by soluble, glutathione-independent lipocalin-type PGD2 synthase. 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 Express ELISA Kit is a competitive assay that can be used for quantification of PGD2 in cell culture lysates and enzymatic reactions. This ELISA offers the convenience of a fast assay with a two hour incubation time and one hour development time, while still achieving an assay range of approximately 117.2-15,000 pg/ml and a sensitvity (80% B/B0) of approximately 350 pg/ml.