Dimeric sesquiterpenoid shizukaol B (SKB) was isolated from Chloranthus japonicus Sieb. Except that SKB inhibited adhesion molecule expression in monocytes and endothelial cells, no more biological and pharmacological activity of SKB had been reported until now. In this study, we examined immunosuppressive activity of SKB. SKB strongly inhibited lipopolysaccharide (LPS)-induced B cell proliferation with $IC_{50}$ of 137 ng/ml, but slightly or not concanavalin A-induced T cell proliferation, LPS-induced macrophage NO production, and LPS-induced dendritic cell maturation. As a mechanism, SKB strongly induced apoptotic death of B cells, but not other cell types. These results suggested that SKB induced toxicity-mediated immunosuppression against B cells.