Quinolone antimicrobial agents induce the SOS response in bacteria, including the umuDC genes necessary for error-prone repair. Consequently these drugs may be mutagenic in bacteria with a functional SOS response. Differential killing tests with Escherichia coli WP2 (trp) and its repair-deficient derivative CM871 (trp lexA recA uvrA) indicated that a functional DNA repair system was protective against the action of quinolones, implying that quinolones are causing some form of DNA damage (not necessarily directly) and are therefore genotoxic. Dose-dependent reversion from His- to His+ with quinolones was observed in the Ames test with Salmonella typhimurium TA102 (uvr+) but in no other Salmonella tester strains (all uvr-), suggesting that a functional excision repair system is essential for quinolone-induced bacterial mutagenesis. A significant correlation between SOS inducing potential (SOSIP) and mutagenic potential in the Ames test (r = 0.89) indicated that quinolone-induced mutagenic effects in bacteria are almost entirely due to SOS-processed DNA damage.