Ceftriaxone (Ro 13-9904), a newly developed cephalosporin with a long half-life, was evaluated for efficacy and safety in 19 patients with serious infections. Underlying illnesses were present in 16 patients. Ceftriaxone was given intravenously every 12 h. Infections treated included gram-negative bacillary pneumonias (two cases), staphylococcal and streptococcal soft tissue-skeletal infections (six cases), spontaneous peritonitis (two cases), and complicated urinary tract infections (nine cases). Bacteremia was present in three patients. Microbiological and clinical cures were achieved in all but one case, although three patients with urinary infection had recurrences 6 weeks posttherapy. The only failure occurred in a patient with pneumonia who had a Pseudomonas aeruginosa isolated from sputum with an initial minimal inhibitory concentration of 4 micrograms/ml, but after 9 days of therapy, a repeat isolate had a minimal inhibitory concentration of 32 micrograms/ml. The minimal inhibitory concentrations for the other isolates ranged from less than or equal to 0.6 to 8.0 micrograms/ml. The mean peak plasma level of ceftriaxone was 99.9 micrograms/ml. The only side effects noted were drug fever in one patient, phlebitis in two patients, and thrombocytosis in four patients.