Rat interleukin-13 (IL-13) is a 14.1 kDa glycosylated immunoregulatory cytokine with two intra-molecular disulfide bonds forming a bundled four α-helix configuration. It is secreted by a variety of immune cells. IL-13 is involved in a number of biological processes, including positive regulation of B-cell proliferation, macrophage activation, immunoglobulin production, protein secretion, and phosphorylation of Stat6 protein. IL-13 initially interacts with IL-13 Rα1 to form a low-affinity complex. The formation of this complex triggers association with IL-4 Rα to form a high-affinity complex that also functions as the type 2 IL-4 receptor complex. IL-13 also binds with high affinity to IL-13 Rα2, which is expressed intracellularly as a soluble protein as well as on the cell surface. It is involved in a number of disorders including allergic rhinitis, inflammatory bowel disease, and colorectal cancer. The Rat IL-13 Kit provides assay-specific components for the quantitative determination of endogenous IL-13 in rat urine, serum, plasma, and cell culture supernatants.