Finafloxacin is a novel fluoroquinolone that exhibits enhanced antibacterial activity under acidic conditions. The aim of this study was to define the in vitro pH-activity relationship. Finafloxacin exhibited optimal antibacterial activity between pH 5.0 and 6.0 at which MICs were 4-to 8-fold lower than those determined at neutral pH. These observations were then confirmed against a larger collection of bacteria. These data suggest that finafloxacin could potentially offer a therapeutic advantage within acidic foci of infection.Fluoroquinolones are a widely utilized class of antibacterial agent. However, a number of attempts to develop new, more potent, members of this class have failed due to concerns over safety and tolerability that have resulted in a halt to development, withdrawal from the market, or restriction of the market (12). Finafloxacin is a new fluoroquinolone belonging to a novel 8-cyano subclass that exhibited a low potential for toxicity or tolerability issues in preclinical tests (14) and in later clinical trials (13). Finafloxacin was also highly effective when tested in in vivo infection models, perhaps more so than would have been predicted from its in vitro MIC (7,8). This effect was