We have analyzed the interleukin-1β (IL-lβ) promoter region near the cap site. Specific DNA sequences required for lipopolysaccharide (LPS) induction within this region were identified using transfection of reporter plasmids that contained portions of the proximal IL-1β 5′-flanking sequence. An LPS-responsive activation area was localized between nucleotides −50 to −100, and down-regulating sequences were present between nucleotides −100 and −2,111. A NFIL-6 site between −92 and −84 was identified in the functionally active region. Base substitutions within this single NFIL-6 site in the context of a 4.1-kb 1L.-1β promoter segment resulted in dramatic reduction of LPS-induced gene transcription. Introduction of multimers of this NFIL-6 sequence immediately 5′ to minimal homologous or heterologous promoters conferred LPS inducibility in each case. Anti-C/EBPβ (NFIL-6) and anti-C/EBPδ (NFIL-6β) antibodies identified both of these proteins in complexes formed between the NFIL-6 site and mononuclear cell nuclear extracts. These data show that the proximal NFIL-6 site is required for the activation of murine IL-1β gene expression by endotoxin. © 1994, Mary Ann Liebert, Inc. All rights reserved.