The cell types of the gut expressing Toll-like receptor 4, which recognizes specifically bacterial lipopolysaccharides, as well as the functionality of this receptor, have remained controversial. We aimed to clarify these issues. Mouse and human intestinal specimens were stained immunohistochemically to detect Toll-like receptor 4 expression. Smooth muscle and myenteric plexus cells but not enterocytes revealed receptor expression. Murine intestinal smooth muscle and myenteric plexus cells but not enterocytes showed nuclear translocation of nuclear factor-kappaB after in vivo stimulation with lipopolysaccharide. Moreover, lipopolysaccharide added to human jejunum biopsies free of epithelial cells induced release of interleukin-8 (IL-8). We can conclude that Toll-like receptor 4 is not expressed in epithelial layer, but rather on smooth muscle and myenteric plexus cells and that expression is functional. The expression of Toll-like receptor 4 on smooth muscle and myenteric plexus cells is consistent with the possibility that these cells are involved in intestinal immune defense; the low or absent expression of Toll-like receptor 4 on enterocytes might explain the intestinal epithelium hyporesponsiveness to the abundance of LPS in the intestinal lumen. J. Cell. Physiol. © 2006 Wiley-Liss, Inc.