The purpose of this study was to investigate the molecular mechanisms that are responsible for the antiinflammatory effect of usnic acid (UA). UA is one of the most common and abundant lichen metabolites. The present study examined the effects of UA on the tumor necrosis factor-alpha (TNF-alpha) and nitric oxide (NO) production induced by lipopolysaccharide (LPS) in RAW264.7 macrophages and the underlying molecular mechanisms. UA decreased the TNF-alpha level in LPS-stimulated RAW264.7 macrophages in dose-dependent manner, the IC(50) value was 12.8 microM. RT-PCR analysis indicated that it inhibited TNF-alpha mRNA expression. Furthermore, it inhibited NO production in LPS-activated RAW264.7 macrophages, the IC(50) value was 4.7 microM. Western blot analysis showed that UA attenuated LPS-induced synthesis of iNOS protein and nuclear translocation of NF-kappaB p65 in the macrophages, in parallel. UA also inhibited LPS-mediated I-kappaBalpha degradation. Taken together, this suggests that UA has an antiinflammatory effect by inhibiting TNF-alpha and iNOS expression, possibly through suppression of nuclear translocation of NF-kappaB p65 and I-kappaBalpha degradation.