Interleukin 1 receptor antagonist (IL-lra) is a protein that binds to the IL-1 receptor and blocks the binding of both IL-la and -.8 without inducing a signal of its own.Human IL-lra has some sequence identity to human IL-1(3, but the evolutionary relationship between these proteins has been unclear. We show that the genes for human, mouse, and rat IL-lra are similar to the genes for IL-la and IL-1(3 in intron-exon organization, indicating that gene duplication events were important in the creation of this gene family. Furthermore, an analysis of sequence comparisons and mutation rates for IL-la, IL-1pj, and IL-lra suggests that the duplication giving rise to the IL-lra gene was an early event in the evolution of the gene family. Comparisons between the mature sequences for IL-lra, IL-la, and IL-1(3 suggest that IL-lra has a P-stranded structure like to IL-la and IL-1iB, consistent with the three proteins being related. The N-terminal sequences of IL-lra appear to be derived from a region of the genome different than those of IL-la and IL-1p3, thus explaining their different modes of biosynthesis and suggesting an explanation for their different biological activities.Interleukin 1 (IL-1) is believed to be important in mediating inflammatory and immune responses (1, 2). IL-1 proteins have been cloned from several species, and in each case two proteins have been identified (3). These two proteins, IL-la and IL-1,3, have the same biological activities and bind to the same cell-surface receptors (4-6).Inhibitors of IL-1 have been reported in the literature over the past several years (7,8). We recently reported the isolation, cloning, and characterization of a cDNA for a protein from human monocytes, IL-lra, that acts as a receptor antagonist on the 80-kDa IL-1 receptor (9, 10). Subsequently, Carter et al. (11) isolated and cloned an IL-1 inhibitor from the human monocyte-like cell line U937 and showed on the basis of its sequence that this molecule is IL-lra. IL-lra appears to be related to the IL-1 family based on its homology and similar hydropathy profile to IL-1,, but unlike the IL-1 proteins, IL-lra has a classical secretory leader peptide and is glycosylated at a consensus N-linked glycosylation site (9, 10).To determine whether IL-1 and IL-lra evolved from a common precursor or whether their similarities are the result of convergent evolution of a structure that can bind to the IL-1 receptor, we isolated genomic clones for IL-lra from human, mouse, and rat and compared their coding sequences with those of IL-la and IL-1,8 from the same and other species.* Our findings support the view that these three proteins have a common ancestor and that the gene leading to IL-lra diverged early in the evolutionary history of the IL-1 family. The results also argue for an important role ofthe sequence around the N terminus of IL-1ra in the unique properties of this protein.