We report that the product of the inducible gene encoding the kinase known as IKKi/IKKepsilon (IKKi) is required for expression of a group of genes up-regulated by pro-inflammatory stimuli such as bacterial endotoxin (lipopolysaccharide (LPS)). Here, using murine embryonic fibroblasts obtained from mice bearing deletions in IKK2, p65, and IKKi genes, we provide evidence to support a link between signaling through the NF-kappaB and CCAAA/enhancer-binding protein (C/EBP) pathways. This link includes an NF-kappaB-dependent regulation of C/EBPbeta and C/EBPdelta gene transcription and IKKi-mediated activation of C/EBP. Disruption of the NF-kappaB pathway results in the blockade of the inducible up-regulation of C/EBPbeta, C/EBPdelta, and IKKi genes. Cells lacking IKKi are normal in activation of the canonical NF-kappaB pathway but fail to induce C/EBPdelta activity and transcription of C/EBP and C/EBP-NF-kappaB target genes in response to LPS. In addition we show that, in response to LPS or tumor necrosis factor alpha, both beta and delta subunits of C/EBP interact with IKKi promoter, suggesting a feedback mechanism in the regulation of IKKi-dependent cellular processes. These data are among the first to provide insights into the biological function of IKKi.