TOC-50, a new parenteral cephalosporin, was assessed for antibacterial activity in vitro and in vivo. In vitro, the activity of TOC-50 was greater than ampicillin, erythromycin and gentamycin, and was equal to that of imipenem. The MICs of TOC-50 for 90% of the clinical isolates tested (MIC90 values) were 0.0125 microgram/ml for penicillin-susceptible Streptococcus pneumoniae, 0.2 microgram/ml for intermediately penicillin-resistant S. pneumoniae, and 0.39 microgram/ml for fully penicillin-resistant S. pneumoniae. In murine systemic-infection models, TOC-50 had a potent protective activity against penicillin-susceptible of fully penicillin-resistant S. pneumoniae. Its protective activity was stronger than that of imipenem.