Interleukin 1 (IL-1) is a polypeptide cytokine produced by various cell types that mediates a broad range of biological activities and possesses a wide spectrum of immunologic and non immunologic properties. Interest has recently focussed on the role of IL-1 as an inflammatory mediator in chronic and acute diseases. The production of IL-1 inhibitors may be a part of a homeostatic mechanism regulating IL-1 effect on immune and inflammatory cell function. Recent reports concerning the purification, characterization, and expression of a cDNA for an IL-1 receptor antagonist (IL-1ra) have been published. Further study of this protein should be valuable in determining the role of IL-1a in man. The protein should also be useful for studying the list of IL-1 activities. Finally, IL-1a is a potentially useful therapeutic agent for the treatment of IL-1-mediated disease in man.