The inhibitory effect of silydianin, an active constituent of Silybium marianum, on the in vitro production and release of oxidative products has been examined. Polymorphonuclear neutrophils (PMNs) play a primary role in the initiation and propagation of inflammatory responses. Their apoptosis is a major mechanism associated with the resolution of inflammatory reactions. Neutrophils were assessed for caspase-3 activity, the rst step in the execution phase of apoptosis. When cells were cultured with 100 µm silydianin for 24 h, caspase-3 was activated. Induction of apoptosis by silydianin was accompanied by a decrease in luminol-enhanced chemiluminescence as well as superoxide radical (O2•−) release in freshly isolated cells and lipid peroxidation in mouse spleen microsomes. No signicant effect of silydianin on PMN hydrogen peroxide production evaluated by a flow cytometric dichlorofluorescin oxidation assay was found. Such results indicate a possible antiinflammatory activity for silydianin, which regulates caspase-3 activation, affects cell membranes and acts as a free radical scavenger. Copyright © 2006 John Wiley & Sons, Ltd.