DNA repair limits the mutagenic, and thereby the carcinogenic, effect of DNA modifications. Free radicals, particularly reactive oxygen species, induce all forms of DNA damage, including base modifications, base free sites, strand breakage, and cross-links. These lesions are repaired by a variety of enzymes of partly overlapping substrate specificity, some of which may be induced. © 1991 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.