Two phenylpropanoids from Bupleurum fruticosum (Apiaceae/Umbelliferae) were shown to inhibit the transcriptional activity induced by PMA or TNFalpha of an NF-kappaB-controlled reporter gene. Western blot experiments indicated that the phenylpropanoids did not prevent IkappaBalpha degradation, suggesting that their molecular target is at a post-IKB degradation level. Both compounds prevented cytokine (IL-1, IL-6, TNF, IL-8) release and prostaglandin E2 synthesis.