IL-1 receptor antagonist(IL-1RA) is a member of the Interleukin 1 cytokine family and a different type of naturally occurring inhibitor of IL-1 activity, it inhibits IL-1α and IL-1β binding to interleukin receptors. A cDNA encoding this polypeptide has been isolated from monocytes and found to code for a mature 152 amino acid residue glycoprotein of 25,000 native molecular weight. This molecule, known as secreted IL-1 receptor antagonist, shows 26% amino acid homology to IL-1βand 19% homology to IL-1α. Evidence indicates that the inhibitory action of sIL-1ra results from binding of IL-1ra to the IL-1 receptor Type I with an affinity comparable to that of IL-1αor IL-1β (Kd ~ 200 pM), thus competing with IL-1α or IL-1β for binding to this receptor. This binding, however, does not result in signal transduction.