Interleukin (IL)-1 is known to be a cytokine which plays a major role in pathological conditions like septic shock, inflammation and auto-immune disease, hence, methods that reduce the activity of IL-1 have an impact on clinical medicine. Inhibiting the binding of IL-1 to IL-1 receptors is one of the methods. A new inhibitor of IL-1 receptor, 139A, was isolated from the fermentation broth of Streptomyces sp.139. It was extracted from the broth filtrate, purified by Diaion HP-20, cation exchange resin and DEAE Sephadex A-25. 139A was identified as polysaccharide, its structure was elucidated on the basis of spectral analysis, the immobilized ligand IL-1 receptor binding assay (IL-ILRBA) proved 139A can competitively inhibits the binding of IL-1 to IL-1 receptors.