The therapeutic efficacy of ciprofloxacin, used alone and in combination with either ceftazidime or gentamicin, was evaluated in a model of experimental Klebsiella pneumoniae septicemia in neutropenic mice. Therapeutic results were compared to those achieved by treatment with ceftazidime or gentamicin alone, as well as their combination. Infections were induced by i.v. injection of two strains of K. pneumoniae. Therapy with i.m. antibiotics was performed in 8 h intervals for a total of 13 doses, and survival rates were recorded at the end of treatment as well as seven days later. Ciprofloxacin alone proved to be significantly more effective than ceftazidime and gentamicin, producing survival rates similar to the combination of ceftazidime plus gentamicin. Bacterial counts of spleens and blood confirmed the superior bactericidal activity of ciprofloxacin. Except for the combination of ciprofloxacin with ceftazidime, there was no apparent advantage of combinations of ciprofloxacin with the other agents compared to ciprofloxacin alone. These data suggest a high in vivo activity of ciprofloxacin in systemic infection due to K. pneumoniae.