Fleroxacin is a new fluoroquinolone with established potent antimicrobial and pharmacokinetic properties, and the aim of this study was to determine its rate of penetration into human lung, muscle, and fat tissues. In a total of eight patients undergoing lung surgery, plasma and tissue concentrations of unmetabolized fleroxacin were determined. This was done in subgroups of two patients each, receiving either 6, 8, 12 or 24 hours prior to surgery one single oral dose of 400 mg fleroxacin. It was found that fleroxacin penetrates well into muscle and lung tissues, but not into fat tissue. The levels in muscle and lung were 1.1 to 4.7 times higher than the according plasma concentrations.