The interleukin-1 (IL-1) family members play an important role in the process of inflammation and host defense. We describe here the identification and characterization of a novel member of the IL-1 family, IL-1HY2. The human IL-1HY2 protein shares significant amino acid sequence similarity (37%) with the IL-1 receptor antagonist and has a predicted three-dimensional structure similar to that of the IL-1 receptor antagonist. The IL-1HY2 gene is located in close proximity to other IL-1 family genes on human chromosome 2, and the genomic organization of the IL-1HY2 gene is highly conserved with other IL-1 family members. IL-1HY2 protein is secreted from mammalian cells, and the purified recombinant IL-1HY2 protein binds soluble IL-1 receptor type I. IL-1HY2 is expressed in human skin, spleen, and tonsil. Immunohistochemical analysis showed that the IL-1HY2 protein is expressed in the basal epithelia of skin and in proliferating B cells of the tonsil. These data suggest that IL-1HY2 is a novel IL-1 family member and that it may participate in a network of IL-1 family members to regulate adapted and innate immune responses.