The in vitro susceptibility of ten strains of Francisella tularensis to four fluoroquinolones (ciprofloxacin, norfloxacin, ofloxacin and pefloxacin) was investigated by determining the MBCs of these quinolones. The results were as follows (mean +/- SE): ciprofloxacin 0.13 +/- 0.03 mg/l, norfloxacin 0.24 +/- 0.07 mg/l, ofloxacin 2.16 +/- 0.78 mg/l and pefloxacin 0.51 +/- 0.50 mg/l. These concentrations can be achieved in clinical practice. In addition, four tularemia patients were treated with an oral regimen of 750 mg ciprofloxacin b.i.d. and one patient with norfloxacin 400 mg b.i.d. The fever experienced by these volunteers vanished within a couple of days and they were able to resume normal work one week after receiving the antibiotics without any relapses later. These in vitro and in vivo results show that orally administered fluoroquinolones are promising antimicrobial agents for the treatment of human tularemia.