The response to three quinolones of a clinical isolate of Streptococcus faecalis was studied by continuous turbidimetric monitoring, by viable counting, and in an in-vitro model of the treatment of bacterial cystitis. Dense populations (c. 8 x 10(6) cfu/ml) responded to ciprofloxacin, norfloxacin and enoxacin at concentrations below the conventionally determined MIC, but variants, shown to exhibit decreased susceptibility, emerged during overnight incubation. The variants were cross-resistant to each of the three quinolones, but the resistance was lost on subculture in drug-free broth. At a concentration of ten-fold MIC, ciprofloxacin was more bactericidal than norfloxacin or enoxacin. In experiments in the bladder model, doses of ciprofloxacin achieving a peak concentration of 10, 50 or 250 mg/l suppressed bacterial growth for 20 h or more. The doses of norfloxacin or enoxacin required to achieve a comparable effect were higher than those of ciprofloxacin. A reduction in susceptibility, as judged by comparative disc testing, occurred after two cycles of exposure to norfloxacin or enoxacin in experiments in which the peak concentration achieved was 50 mg/l, but not when the peak was 250 mg/l. Reduced susceptibility to ciprofloxacin developed only in experiments in which a lower peak concentration of 10 mg/l was achieved.