The objective of this study was to evaluate the in-vitro activity of levofloxacin, ofloxacin and D-ofloxacin compared with ciprofloxacin, norfloxacin and sparfloxacin against coryneform bacteria and Listeria monocytogenes isolated from clinical samples. The following organisms (and number of strains) were studied: Corynebacterium jeikeium (20), Corynebacterium urealyticum (20), Corynebacterium minutissimum (20), Corynebacterium striatum (20), Corynebacterium amycolatum (30), Brevibacterium spp. (15) and Listeria monocytogenes (15). Antimicrobial activity was determined by microdilution using cation-adjusted Mueller-Hinton broth supplemented with 0.5% Tween 80 when testing C. jeikeium or C. urealyticum. Fluoroquinolones were used in the range 0.015-16 mg/L. Plates were incubated in air at 35 degrees C for 18-20 h (24 h when testing C. jeikeium or C. urealyticum). The following MIC50 values were obtained for all 140 organisms tested: levofloxacin, 1 mg/L; ofloxacin, 2 mg/L; D-ofloxacin, > 16 mg/L; ciprofloxacin, 1 mg/L; norfloxacin, 16 mg/L; sparfloxacin, 1 mg/L. MIC90 values were > 16 mg/L for all test antibiotics with the exception of levofloxacin, which had an MIC90 value of 16 mg/L. At a concentration of 2 mg/L, levofloxacin inhibited all L. monocytogenes strains and 35-93% of the remaining species. MIC90 values of ofloxacin were one dilution step higher than those of levofloxacin against C. minutissimum, C. striatum, Brevibacterium spp. and L. monocytogenes. Levofloxacin showed similar (C. jeikeium, C. urealyticum, C. amycolatum and Brevibacterium spp.) or greater (C. minutissimum and C. striatum) activity than ciprofloxacin and sparfloxacin, and higher than D-ofloxacin or norfloxacin against all species studied. In conclusion, levofloxacin was the most active of the six fluoroquinolones evaluated against coryneform bacteria isolated from clinical samples and could therefore be a promising treatment option in this setting.