The effects of ofloxacin, ciprofloxacin and balofloxacin on the reactive oxygen species (ROS) levels generated by human neutrophils was examined in vitro; ROS generated in a cell-free, xanthine-xanthine oxidase system was also assessed. The species investigated were superoxide radical anion (O2-), hydrogen peroxide (H2O2) and hydroxyl radical (OH*). Both ofloxacin and ciprofloxacin markedly decreased the levels of O2-, H2O2 and OH* generated by human neutrophils. On the other hand, these drugs did not affect any of the ROS examined in the xanthine-xanthine oxidase system. Balofloxacin showed no significant effect on ROS generated by either system. The present study indicates that ofloxacin and ciprofloxacin may exert an anti-inflammatory action by reducing the potent ROS species excessively generated by neutrophils at the sites of inflammation.