Interleukin (IL)-33 is the latest member of IL-1 cytokine family. In this study, the cloning, expression, purification, and polyclonal antibody preparation of mouse IL-33 were described. The coding region of IL-33 mature protein was cloned into the prokaryotic expression vector pET-44. The recombinant protein, IL-33 containing a hexahistidine tag in the C-terminal, was expressed in Escherichia coli. The expressed soluble protein was purified by immobilized metal-ion affinity chromatography using [Ni.sup.2+]-nitrilotriacetic acid agarose. The rabbits were immunized with the purified recombinant protein. The obtained antiserum was precipitated by saturated ammonium sulfate and then purified by Protein A affinity chromatography. The sensitivity and specificity of the antibodies were evaluated by enzyme-linked immunosorbent assay and immunohistochemistry. The high titer (1 : 32000) polyclonal antibodies with high specificity were obtained by immunizing rabbits with the purified recombinant protein. Significant expression of IL-33 was seen in mouse liver and lung tissues determined with the anti-IL-33. The production of the polyclonal antibody against IL-33 provides a good tool for studying the biofunctions of IL-33. Copyright (C) 2009 Xiaojin Liu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited., 1. Introduction The classical interleukin (IL)-1 family cytokines, IL-1α and IL-1β, as well as IL-18, are related by mechanism of origin, +receptor structure, and signal transduction pathways utilized [1]. IL-33 [...]