Silybin has a potent antibacterial activity, more potent than silymarin II, against gram-positive bacteria without hemolytic activity, whereas it has no antimicrobial activity against gram-negative bacteria or fungi. The mode of action of silybin against the gram-positive bacterial cell was examined by investigating the change in plasma membrane dynamics of bacterial cells using 1 ,6-diphenyl-1,3,5-hextriene (DPH) as a membrane probe and by assessing the inhibition of macromolecular synthesis using radiolabeled incorporation assay. The results showed that silybin inhibited RNA and protein synthesis on gram-positive bacteria.