Silybin (silibinin) is the major flavonolignan from the extracts of milk thistle seed, Silybum marianum. The whole extract, known as silymarin, has been found to protect the liver from both acute and chronic toxicity and injury. Silybin blocks the production of superoxide in PMA-activated rat Kupffer cells with an EC50 of around 100 µM.1 Silybin also blocks the synthesis of leukotriene B4 by these same cells with an IC50 of about 15 µM. Thus, part of the protective effects of silybin could be due to 5-lipoxygenase inhibition.