Objective . Levonadifloxacin is a broad-spectrum anti-staphylococcal drug that is under development. We investigated the in vitro activity of levonadifloxacin against methicillin-susceptible Staphylococcus aureus (MSSA) and methicillin-resistant S. aureus (MRSA) strains phagocytized in THP-1 monocytes to evaluate its scope for treatment of intracellular staphylococcal infections.Methods. The microdilution minimum inhibitory concentrations (MICs) of levonadifloxacin, moxifloxacin, levofloxacin and ciprofloxacin against MSSA ATCC 25923 and MRSA ATCC 43300 strains at pH 7.4±0.1 (original medium pH) and 5.5±0.1 (phagosome pH environment) were determined by following Clinical and Laboratory Standards Institute (CLSI) guidelines. The activity of antibiotics was investigated by extracellular and intracellular time-kill studies at 1-16× MIC concentrations. A suspension of ~5× log10 c.f.u. ml-1 test organism in supplemented RPMI 1640 medium was employed to determine the extracellular activity, while test organism phagocytized at a 4 : 1 ratio of bacteria to THP-1 monocytes was employed to investigate the intracellular activity. At intervals of 0, 2, 6 and 24 h, colony-forming unit (c.f.u.) counts were performed in triplicate on inoculated brain heart infusion (BHI) agar plates for both methods.Results. At pH 7.4, the MIC of levonadifloxacin against both tested S. aureus strains was 2, 8 and 16 times lower than those of moxifloxacin, levofloxacin and ciprofloxacin, respectively. At pH 5.5, the MIC of levonadifloxacin was reduced by ≥8× against both tested S. aureus strains compared to its MIC at pH 7.4. In contrast, comparator quinolones showed a fourfold elevation in MIC at pH 5.5. In the study assessing the extracellular bactericidal effect, levonadifloxacin at 1× MIC manifested ≥4.5 log10 c.f.u. ml-1 killing for both S. aureus strains. Moxifloxacin and levofloxacin also showed bactericidal activity, while ciprofloxacin showed no killing. In intracellular conditions, levonadifloxacin manifested 1.0 log10 and 2.0 log10 killing for intracellular S. aureus ATCC 25923 and ATCC 43300, respectively. These killing effects were better overall than those of comparator quinolones.Conclusions. Within a clinically achievable concentration range, levonadifloxacin achieved a 90-99 % intracellular reduction of MSSA and MRSA strains phagocytized in THP-1 monocytes. Therefore, levonadifloxacin has the potential to be a therapeutic option for the management of intracellular methicillin- and quinolone-resistant staphylococcal infections.