The interleukins are a broad family of well characterized cytokines, primarily of hematopoietic cell origin. Cytokines are small, soluble proteins with pleiotropic effects on a variety of cell types. Cytokines have a regulatory function over the immune system and mediate aspects of inflammatory response (1,2). IL-25 (also called SF20/IL-25) is a secreted bone marrow stroma-derived growth factor cytokine that is derived from Th2 T-cells (3,4). IL-25 belongs to the IL-17 cytokine family and is capable of amplifying allergic type inflammatory responses by its actions on other cell types (4,5). IL-25 binds to mouse thymic shared antigen-1 and supports lymphoid cell proliferation (3). Infusion of mice with IL-25 induces IL-4, IL-5, and IL-13 gene expression (4,5). IL-25 induced mice also develop epithelial cell hyperplasia, increased mucus secretion, and airway hyperreactivity (4,5), suggesting that IL-25 may be an important mediator of allergic disease via production of IL-4, IL-5, IL-13 and eotaxin (5).