We cloned the 5' upstream region of IL-15 genomic DNA and examined promoter activity in macrophages stimulated with lipopolysaccharide(LPS). The 1.2 kilobase (kb) fragment of the 5' upstream region contained binding elements for LPS-inducible transcription factors such as NFIL-6 or NF-kappaB. Determined by luciferase assay following transient transfection in the J774A.1 macrophage cell line, the 1.2 kb of the 5' upstream region exhibited high promoter activity in response to LPS, while promoter activity was significantly reduced by the 5' deletion of 313 base pairs containing the NF-kappaB binding motif. Nuclear protein prepared from LPS-stimulated macrophages formed a complex with the NF-kappaB binding sequence of the IL-15 promoter. Taken together, the binding of nuclear protein to the NF-kappaB binding site is required for transcriptional activation of the IL-15 gene in LPS-stimulated macrophages.