Interleukin (IL)-1-like protein 1 (IL-1L1) is a 155-amino acid protein that shares 27% identity with IL-1beta and 47% with IL-1 receptor antagonist (IL-1ra). A 2.7-kb IL-1L1 mRNA was cloned from human placenta and is detectable in the trophoblastic cell line JEG-3, in macrophages and in endotoxin-stimulated monocytes. Expression of IL-1L1 is much less abundant and less widespread than IL-1ra. We have determined the human and mouse IL-1L1 cDNA sequences and the complete sequence of the human gene, IL1L1. IL1L1 consists of four coding exons, has two alternative non-coding first exons, lies between IL1B and IL1RN, is orientated in the same direction as IL1RN and is separated from it by approximately 53 kb. The predicted IL-1L1 protein lacks both signal sequence and glycosylation signals. A 17-kDa protein was recovered by immunoprecipitation with IL-1L1-specific antibodies from JEG-3. IL-1L1 did not stimulate IL-6 production from primary human fibroblasts or human umbilical vein endothelial cells nor did it block the IL-1alpha or IL-1beta-dependent activation of IL-6 expression. We conclude, contrary to a recent suggestion made by others, that IL-1L1 is not a functional IL-1ra. IL-1L1 also had no detectable agonistic or antagonistic effect on IFN-gamma production in response to IL-18 in KG-1 cells.