Pharmacological inhibition of interleukin-12 (IL-12) production may allow a therapeutic strategy for preventing development and progression of disease in experimental models of autoim-munity. In this study we investigated the effects of an ethanol fraction of the Scapharca broughtonii, on the production of IL-12 by mouse macrophages stimulated with lipopolysaccha-rides (LPS). The ethanol fraction (S3) prepared from Scapharca broughtonii potently inhibited LPS-induced IL-12 production in the RAW264.7 monocyte cell-line in a dose-dependent manner. The activation effect of the ethanol fraction (S3) on the IL-12 gene promoter was analyzed by transfecting RAW264.7 cells with IL-12 gene promoter/luciferase constructs. The repressive effect mapped to a region in the IL-12 gene promoter that contained a binding site for NF-κB. Furthermore, activation of macrophages by LPS resulted in markedly enhanced binding activity to the NF-κB site, which significantly decreased upon addition of the ethanol fraction, indicating that the ethanol fraction of the blood shell inhibited IL-12 production in LPS-acti-vated macrophages via inhibition of NF-κB binding activity.