A total of 2865 strains of the causative organisms isolated from the patients with acute pharyngitis and tonsillitis at the primary medical institutions were used in this study. The MICs of levofloxacin (LVFX) and other oral antimicrobial drugs were determined and evaluated by the NCCLS guideline. LVFX, cefditoren (CDTR) and cefcapene (CFPN) were potently active against 773 isolates of Hemophilus influenzae, the MIC50S of LVFX being < or = 0.06 microgram/mL and also the same as the MIC90S of LVFX. LVFX was the most active against 496 isolates of Enterobacteriaceae. The MIC50S of LVFX were < or = 0.06 microgram/mL and were lower than those of CDTR, cefdinir (CFDN) and cefpodoxime (CPDX) (MIC50S: 0.5 microgram/mL). The MIC90S of these cephems were markedly higher than the respective MIC50S, whereas MIC50 of LVFX was 0.12 microgram/mL, only twice the MIC50. Against the majority of Streptococcus pyogenes (555 isolates) and Streptococcus spp. (495 isolates), CDTR, CFDN, CPDX and CFPN were highly active (MICs: < or = 0.06 microgram/mL), and clarithromycin (CAM) and azithromycin (AZM) were also active against these organisms (MICs: 0.12 to 0.25 microgram/mL). Against S. pneumoniae (92 isolates), CDTR and CFDN were active (MIC50S: 0.12 and 0.25 microgram/mL, respectively). However, the MIC90S of these drugs were 4-8 times the MIC50S. Against Moraxella (Branhamella) catarrhalis (454 isolates), LVFX was potently active, the MIC90 of LVFX being < or = 0.06 microgram/mL and MIC90S of the other cephems being 0.5 microgram/mL or more. When the susceptibility of these strains to LVFX was evaluated by the NCCLS guideline, about 3% of other Streptococcus spp. were resistant to the drug but no test strains resistant to LVFX were detected in H. influenzae, S. pyogenes or Enterobacteriaceae. On the other hand, the percentages of strains susceptible to the cephems tested were 60-90%, which were quite different according to kinds of drugs and species used. Furthermore, the strains of S. pneumoniae resistant to CFDN and CPDX, and those to CAM and AZM were 21-25% and 50% or more, respectively, whereas no LVFX-resistant strains were detected. The major pathogens isolated from patients with pharyngitis and tonsillitis in the primary institutions were highly susceptible to LVFX. These results suggest that LVFX is a useful drug which is potently active against the strains resistant to oral cephem and macrolide antibiotics.