It has been evaluated the effect of norfloxacin, ofloxacin, levofloxacin, fleroxacin and lomefloxacin of the respiratory burst of human polymorphonuclear leukocytes and their intracellular activity against Staphylococcus aureus. None of these agents significantly affected the superoxide and hydrogen peroxide radicals production by the polymorphonuclear leukocytes. All the quinolones except norfloxacin showed significant intracellular activity against S. aureus.