The purpose of this study was to investigate the in-vitro bacteriostatic activity of levofloxacin in comparison with that of ofloxacin, sparfloxacin and ciprofloxacin against 205 strains of Streptococcus pneumoniae (101 penicillin-susceptible, 51 penicillin-intermediate and 53 penicillin-resistant). The isolates were provided between September 1996 and October 1996 by French hospitals participating in the National Co-operative Survey of Pneumococcal Infections. The determination of MICs (mg/L) was made by the agar dilution method. The MIC50 and MIC90 values of the four fluoroquinolones for the three classes of S. pneumoniae (penicillin-susceptible, penicillin-intermediate and penicillin-resistant) were not significantly different. In contrast, the differences in in-vitro activity observed among the four fluoroquinolones against the 205 strains allowed them to be separated into three groups: sparfloxacin (MIC50/90 0.25 mg/L); ciprofloxacin and levofloxacin (MIC50 0.5 and 1 mg/L respectively, MIC90 1 mg/L); and ofloxacin (MIC50 1 mg/L, MIC90 2 mg/L). A total of 204 of the strains had a levofloxacin MIC between 0.25 mg/L and 1 mg/L, and only one of the 205 strains was highly resistant (MIC 16 mg/L). Whatever the level of susceptibility to penicillin, the relative bacteriostatic activity was, in descending order of activity, sparfloxacin, levofloxacin/ciprofloxacin and ofloxacin. These results suggest levofloxacin has potential for the treatment of pneumococcal infections.