Incubation of free 3,4-dihydroxyphenylalanine (DOPA), protein-bound DOPA (PB-DOPA) and related catechols with DNA, proteins and lipids has been shown to result in oxidative damage to the target molecule. This article reviews these reactions with particular emphasis on those that occur in the presence of molecular O(2) and redox-active metal ions (e.g. Fe(3+), Cu(2+), Cr(6+)), which are known to increase the rate of DOPA oxidation. The majority of oxidative damage appears to be mediated by reactive oxygen species (ROS) such as superoxide and HO(.) radicals, though other DOPA oxidation products, including semiquinone radicals, quinones, and metal ion-DOPA complexes have also been implicated in some cases. Non-radical reactions of DOPA with suitable nucleophiles (e.g. thiol groups) can also result in modification of the target, with this process being particularly prevalent with proteins. The exacerbation of damage observed on addition of H(2)O(2) is in accord with a key role for ROS in many of these reactions.