Exposure of macrophages to lipopolysaccharide (LPS) induces release of tumor necrosis factor-␣ (TNF-␣), which is initially synthesized as a 26-kDa pro-TNF-␣ followed by proteolytic processing to a 17-kDa secreted form. In this study, justicidin A, an arylnaphthalide lignan isolated from Justicia procumbens, was found to inhibit LPS-stimulated TNF-␣ release from RAW 264.7 macrophages in a concentration-and time-dependent manner, and the underlying mechanism was investigated. In the presence of justicidin A, challenge with LPS increased the steady-state level of the 26-kDa membrane-bound form of TNF-␣ protein, whereas justicidin A had little effect on the expression of TNF-␣ mRNA and on the synthesis of pro-TNF-␣ protein. Results of the pulse-chase experiment, revealed that the conversion of pro-TNF-␣ to mature TNF-␣ was inhibited by justicidin A. Moreover, justicidin A suppressed the transport of TNF-␣ to cell surface as analyzed by flow cytometry. The immunofluorescence analysis demonstrated that large amounts of LPS-induced TNF-␣ accumulated primarily within Golgi complex. These results indicate that justicidin A inhibits TNF-␣ release at the step of transport of pro-TNF-␣ to cell surface, and this leads to the accumulation of TNF-␣ in Golgi complex in RAW 264.7 macrophages.