The present invention relates to a novel soluble IL-1 receptor accessory molecule (IL-1R AcM) protein which is a member of the Ig superfamily. In particular, isolated nucleic acid molecules are provided encoding the human IL-1R AcM protein IL-1R AcM polypeptides are also provided as are vectors, host cells and recombinant methods for producing the same. Screening methods are further provided for identifying agonist and antagonists of IL-1 signal transduction. The invention further relates to methods for treating physiologic and pathologic disease conditions with IL-1R AcM antagonists.