In the process of developing compounds to counteract the damaging effects of free radicals in biological systems it is important to determine the type and the intracellular location of specific toxic events. For that purpose cultured cells are used, because a properly designed cell culture system allows to asses specific damage at the cellular and subcellular level and can contribute to an evaluation of the biochemistry of free radical damage. A wide range of changes in cellular activities resulting from oxidative injury in vitro have been demonstrated. Data from many laboratories indicate that cell culture system coupled with appropriate analytical techniques can be used to explore cellular and biochemical details of damage induced by free radicals. The type of reactive oxygen species used to generate the radicals, the rate of radical production, and the location of action of the toxic species must be taken into account to understand the biochemistry of the system. An effort is made to analyse the considerable progress made in the development of appropriate in vitro models and end-points for use in testing and characterising the nature and location of free radical cytotoxicity.