Ceftriaxone is a new semisynthetic cephalosporin with broad-spectrum in vitro activity and an unusually long serum half-life. The clinical efficacy of ceftriaxone was evaluated in 35 infections in 34 patients; 12 of these patients had skin and soft tissue infections, 10 had infections of the urinary tract, 8 had pneumonia, 2 had biliary tract infections, 1 had sinusitis, 1 had diverticulitis, and 1 had a retroperitoneal abscess. Of the 35 infections, 9 were bacteremic. The bacteria isolated included Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus pneumoniae, Streptococcus faecalis, other streptococcal species, Escherichia coli, Proteus mirabilis, Klebsiella pneumoniae, Serratia marcescens, Enterobacter cloacae, Haemophilus influenzae, Pseudomonas aeruginosa, Bacteroides fragilis, other Bacteroides species, and anaerobic cocci. Improvement or cure occurred in 32 episodes, for a response rate of 91%. There were three treatment failures in patients with soft tissue infections. No serious drug toxicities were observed. At a dosage regimen of 1 g every 12 h the peak and trough serum antibiotic concentrations were well above the minimal inhibitory concentrations of most pathogens. Our findings suggest that ceftriaxone is a safe and effective antibiotic for therapy of serious bacterial infections.