Eugenol exhibits antioxidant and anti-inflammatory activities, but at higher concentrations acts as an oxidant and potent allergen. It was earlier shown that bis-eugenol synthesized by the oxidation of eugenol was less cytotoxic and more highly antioxidative than eugenol. But its anti-inflammatory mechanism remains yet unclear. Since nuclear factor-kappa B (NF-kappa B) is a key transcriptional factor in the expression of inflammatory cytokines, we examined whether eugenol and bis-eugenol are inhibitors of NF-kappa B activation. We observed that bis-eugenol, but not eugenol, clearly inhibited the degradation of inhibitory kappa B-alpha in RAW264.7 murine macrophages stimulated with lipopolysaccharide and, consequently, the transcriptional activity of the stimulated NF-kappa B in the cells. In addition, bis-eugenol actually inhibited LPS-stimulated expression of inflammatory cytokines at both gene and protein levels. These findings suggest that bis-eugenol acts as a potent inhibitor of NF-kappa B.