The susceptibility of clinical strains isolated from patients with infection to ceftriaxone (CTRX) and injectable beta-lactam antibiotics was determined in Japanese medical institutions in 2004. The in vitro antibacterial activity of the above antibiotics against clinical strains isolated in 2004 was compared with that against part of the clinical strains isolated about ten years ago (1994 - 1996). The antibacterial activity of CTRX against Streptococcus including penicillin-intermediate and penicillin-resistant Streptocuccus pneumoniae was potent and there were no large differences between clinical strains isolated in 1994 - 1996 and those in 2004 in the antibacterial activity of CTRX. CTRX inhibited the growth of all the strains of Haemophilus influenzae including beta-lactamase-negative, ampicillin-resistant (BLNAR) strains at 0.5 microg/mL or less and showed the potent antibacterial activity against these strains. In addition, since there are no large differences between strains isolated in the past and those isolated in 2004 in the antibacterial activity, CTRX is considered to be a useful antibiotic in the treatment of BLNAR infections which are increased by the causative organism of infections especially in the field of pediatrics in recent years. CTRX showed the excellent antibacterial activity against Escherichia coli and Klebsiella pneumoniae but one of the 50 strains of E. coli tested was highly resistant. The antibacterial activity of CTRX against Neisseria gonorrhoeae was the most potent in all the antibiotics tested. These results indicate that CTRX shows excellent antimicrobial activity against fresh strains isolated from patients with infections. CTRX is thus useful as a therapeutic antimicrobial for the treating a variety of infections.