We compared the antimicrobial activity of commercially available oral cephem agents, cefaclor (CCL), cefroxadine (CXD), cefdinir (CFDN), cefixime (CFIX), cefpodoxime (CPDX), cefteram (CFTM), cefcapene (CFPN), and cefditoren (CDTR), against Streptococcus pneumoniae, Haemophilus influenzae, Escherichia coli, Klebsiella pneumoniae, Staphylococcus aureus, Streptococcus agalactiae, Streptococcus pyogenes, and ESBL-producing bacteria isolated from clinical materials in Kansai Medical University Hospital between 2002 and 2003. Based on the Pharmacokinetics/Pharmacodynamics (PK/PD) theory, we determined the concentration of each agent at which the time above MIC (TAM) value was 40% or more, and calculated the rate of efficacy against each type of bacteria. In S. pneumoniae strains, the MIC(50,80,90) values of CDTR were 0.25, 0.5, and 0.5 microg/ml, respectively, lower than those of the other agents, demonstrating the most potent antimicrobial activity. However, the efficacy rate for CDTR calculated based on the PK/PD theory was 58.5%. CFTM showed the highest efficacy rate (66.1%). In H. influenzae strains, the antimicrobial activity of CDTR was most potent, followed by that of CFTM and that of CFPN/CFIX. The MIC90 value of CDTR was lowest (0.25 microg/ml), followed by that of CFTM (0.5 microg/ml). The efficacy rate for CDTR was 100%. This result supports that CDTR frequently eradicates H. influenzae. In E. coli strains, the MIC90 values of the above agents, excluding CCL and CXD, ranged from 0.5 to 1 microg/ml. The antimicrobial activity of CFIX against K. pneumoniae was most potent, followed by that of CFDN/CPDX and that of CFTM. In ESBL-producing bacteria, most agents showed an MIC90 value of more than 4 microg/ml. In S. agalactiae and S. pyogenes strains, all of the agents showed satisfactory MIC values. In methi- cillin-sensitive Staphylococcus aureus (MSSA) strains, CFDN and CXD showed a high efficacy rate, whereas the efficacy rates for the other agents were low. The frequent use of oral agents has increased the number of cephem-resistant bacteria. ESBL-producing bacteria become highly resistant, and the presence or absence of response can be readily evaluated. However, when a mutation of penicillin-binding protein (PBP) occurs, drug resistance is less marked. Therefore, it is difficult to evaluate the treatment response in many cases. In S. pneumoniae strains, the efficacy rates for all of the agents were low in the evaluation using the PK/PD theory, suggesting that a dose higher than the standard dose should be established. Thus, in the future, the efficacy should be evaluated based on the PK/PD theory, appropriate antimicrobial treatment should be administered, and the administration method that does not increase the number of resistant bacteria must be established.