Itisonlyrecently thatthe naturaloccurrenceoffree radicalsin biological tissue has become widely accepted, and that the suspi cion with which biologists previously viewed the free radicals of radiationchemistryhas beenplacedin a broaderperspective. Now, oxygen-derived free radicals are considered respectable biochemi cal intermediates, given always the caveat that unwanted tissue damage may arise if these active species are produced in such abundance that they overwhelm the natural antioxidant and free radical defense mechanisms, or if these systems have become hypoeffective. Many factors,...