Platelets have the capacity to generate oxygen-derived free radicals and are often present at inflammatory foci with other free-radical-generating cells such as white blood cells. Free radicals can modify platelet adhesion and aggregation directly or through effects on the vascular endothelium, which generates prostacyclin and nitric oxide. To defend against the overproduction of free radicals the body manufactures endogenous scavengers, which can be of enzymic or non-enzymic origin. Daniela Salvemini and Regina Botting describe how free-radical scavengers may be used therapeutically to regulate the platelet reactivity involved in many pathological phenomena.