Sixteen natural phenolics and semisynthetic derivatives thereof, including silymarin, flavonoids, catechines and phenolic acids, together with 6 standard drug substances with anti-inflammatory, antioxidant and peroxide radical scavenging properties have been tested in an in vitro model using human platelets for their inhibitory action against N-ethyl maleimide-induced lipid peroxidation. The malondialdehyde formed during the reaction was estimated by means of the thiobarbituric acid method. In all compounds tested, the inhibitory action was clearly dose-dependent; IC50 values were calculated from the dose-activity curves. The IC50 found ranged from between 1.47 nM and approximately 900 mM.