In vitro susceptibility patterns of Streptococcus pneumoniae to fluoroquinolones in Hong Kong: relationship to serotype and penicillin G resistance. The minimum inhibitory concentrations (MICs) of 26 strains of S. pneumoniae (all susceptible to penicillin G by disc diffusion) against five antimicrobial agents (penicillin, penicillin/ampicillin, cefaclor, clarithromycin and a newer generation fluoroquinolone, gemifloxacin) were determined by an agar dilution method. The study confirmed the trend of increasing fluoroquinolone MICs of S. pneumoniae as resistance to penicillin G increases. The S. pneumoniae serotypes did not have any significant effect on the MICs of penicillin G and fluoroquinolones. Nevertheless, four strains of S. pneumoniae with elevated resistance to penicillin G were not resistant to the five fluoroquinolones tested. In summary, the findings highlight that the in vitro resistance patterns of S. pneumoniae against fluoroquinolones can be different from the resistance patterns against penicillin G, especially when there is no resistance to the fluoroquinolone used. However, fluoroquinolones seem to have good activity against penicillin G susceptible strains. The trend of increasing MICs of fluoroquinolones in penicillin G resistant isolates, but not in susceptible isolates, indicates that the in vitro susceptibility patterns of the newer generation fluoroquinolones against penicillin G susceptible and resistant pneumococci can be quite different. Fluoroquinolone resistance among S. pneumoniae may not result in cross resistance to all fluoroquinolones. It should be stressed that the resistance patterns of S. pneumoniae against fluoroquinolones can vary according to geographical location. The newer fluoroquinolones have excellent activity against penicillin G susceptible S. pneumoniae and the in vitro patterns of activity against penicillin G resistant strains of pneumococci are much better than earlier generations of the fluoroquinolones.