Mast cells are important not only in allergic reactions, but also in inflammation and are involved in a variety of responses including the immediate release of potent inflammatory mediators after activation by cross-linking of FCERI molecules. Prostaglandin D2 (PGD2) is a major cyclooxygenase metabolite of arachidonic acid produced by mast cells and it is released following allergen challenge in allergic diseases. IL-33 is an iflammatory cytokine which is critically involved in the regulation of in vitro and in vivo cyclooxygenase production, providing a potential therapeutic target for inflammatory disorders. In this study, using human derived umbelical cord blood mast cells, we show that IL-33 (50 ng/ml), and calcium ionophore A 23187 (0.5 f.1g/ml), compound 48/80 (10-5 M) or anti-IgE (10 ug/rnl), enhaced the production ofPGD2 and this effect was inhibited by indomethacin. However, IL-33 was unable to induce tryptase release in these cells. These effects confirm the inflammatory property of IL-33 by stimulating PGD2 but not tryptase in human mast cells. The inhibitory effect of this new cytokine may have a potential therapeutic response in allergic and inflammatory diseases.Cytokines are inflammatory and immuneregulatory proteins that play a central role in the pathogenesis of allergic diseases (1-9) and inflammation (10-14). However, some cytokines are anti-inflammatory such as IL-4, IL-l 0, IL-l receptor antagonist (IL-1RA) and IL-37. Cytokines of the interleukin-l (IL-l) family, such as IL-I alpha/beta, IL-18 and lL-33, have also an important function in host defence, immune regulation, and inflammation (15)(16)(17)(18)(19)(20). The mediators released by mast cells, such as histamine, tryptase, serotonin and arachodonic acid products: PGD 2 , LTC 4 , LTD 4 , LTE 4 induce, in general, vasodilation, ronchoconstriction, cellular chemotaxis and increased vascular permeability (21-25). Most of the eicosanoids generated by mast cells are either PGD 2 or LTC 4 (26-29).