The frequency with which Escherichia coli mutated to resist a series of ten 4-quinolone antibacterials was studied. It was found that the mutation frequency could not be predicted from the potency of the drugs against sensitive bacteria. The mutation rates were lowest with ofloxacin, norfloxacin and ciprofloxacin. No mutants were observed with ofloxacin while less mutants were observed with norfloxacin than with ciprofloxacin, despite the latter possessing greatest activity against susceptible bacteria. The mutation frequencies observed with nalidixic acid, cinoxacin, pipemidic acid, flumequine, rosoxacin, oxolinic acid, and enoxacin were much higher than those observed with the three 4-quinolones mentioned earlier. When the sensitivities of the mutants to 4-quinolones were investigated, it was again found that the potency of each 4-quinolone against sensitive bacteria did not correlate with the levels of resistance of mutants to that drug. However, when the relative activities of the 4-quinolones against the mutants were investigated, it was found that no mutant isolated resisted a concentration exceeding the peak serum levels of ciprofloxacin, norfloxacin or ofloxacin. Although some mutants exhibited more resistance than the maximum attainable serum levels of the other 4-quinolones, the sensitivity of all the mutants fell well within the urine levels of pipemidic acid, flumequine, rosoxacin, oxolinic acid, enoxacin, norfloxacin, ofloxacin, and ciprofloxacin.