DNA breakdown occurred in Escherichia coli KL16 exposed to nalidixic acid, ciprofloxacin or norfloxacin. However DNA breakdown does not seem to be the cause of the lethality of the 4-quinolones because it still occurred under conditions which abolished the lethality of nalidixic acid. Furthermore, no correlation was found between the amount of DNA breakdown and the rate of death of bacteria caused by the three 4-quinolones. Similarly, DNA breakdown did not occur when recB or recC mutants were treated with nalidixic acid despite both mutants being killed by the drug, again suggesting that DNA breakdown is not the cause of bacterial death. Since recB and recC mutants lack exonuclease V, this enzyme may be responsible for the DNA breakdown observed in bacteria treated with 4-quinolones.