The in vitro susceptibility of 38 strains of Francisella tularensis (biovar F. tularensis palaearctica) was determined using Etests on cysteine heart agar plates with 2% haemoglobin. All strains were susceptible to the antibiotics traditionally used to treat tularaemia, such as streptomycin (MIC(90) 4.0 mg/L), tetracycline (MIC(90) 0.38 mg/L) and chloramphenicol (MIC(90) 0.38 mg/L), and to aminoglycosides, such as tobramycin (MIC(90) 1.5 mg/L) and gentamicin (MIC(90) 1.0 mg/L). The quinolones examined had low MIC(90)s: ciprofloxacin, 0.016 mg/L; levofloxacin, 0.016 mg/L; grepafloxacin, 0.047 mg/L; and trovafloxacin, 0.032 mg/L. In contrast, all the strains were resistant to beta-lactams and azithromycin. Quinolones thus seem to be promising drugs for the treatment of tularaemia.