The antimicrobial peptide hepcidin appears to play a central role in the regulation of iron homeostasis. In intact animals, iron overload or the injection of lipopolysaccharide (LPS) stimulates transcription of HAMP, the gene that encodes hepcidin. In isolated hepatocytes, IL-6, an inflammatory cytokine the production of which is stimulated by LPS, up-regulates transcription of hepcidin. In contrast, iron has no stimulatory effect on hepcidin expression in isolated hepatocytes. There is apparently a signaling pathway, activated by iron, that is present in the intact animal but not in isolated hepatocytes. Studies in humans and mice have shown that this iron-dependent pathway requires the presence of Hfe, hemojuvelin, and probably transferrin receptor 2 (tfr-2). To determine whether activation of hepcidin transcription by IL-6 also requires Hfe and tfr-2, we have studied mice homozygous for targeted disruption of HFE, β₂-microglobulin, and for a truncating mutation of TFR-2. We show that these mutant mice react normally to injection of endotoxin and that their isolated hepatocytes react normally to IL-6. This indicates that the signaling pathway activated by IL-6 does not require either Hfe or tfr-2. Mice with disruption of the gene encoding IL-6 seem to have a blunted response to LPS, but the statistical significance of the small response documented is borderline. It is therefore not clear whether LPS stimulates secretion of cytokines other than IL-6 that may stimulate hepcidin transcription.