Lascufloxacin exhibited a broad spectrum of activity against various clinical isolates. Furthermore, lascufloxacin showed the most potent activity against Gram-positive bacteria among the quinolones tested and incomplete cross-resistance against existing quinolone-resistant strains. Enzymatic analysis indicated that lascufloxacin had potent inhibitory activity against both wild-type and mutated target enzymes. These results suggest that lascufloxacin may be useful in treating infections caused by various pathogens, including quinolone-resistant strains.KEYWORDS antimicrobial activity, antimicrobial agents, quinolones R ecently, the emergence of strains resistant to commercially available antibacterial drugs, including -lactams, macrolides, tetracyclines, and aminoglycosides, has become a clinical problem around the world. Quinolones are also widely used for treatment of various infectious diseases, and the development of quinolone resistance in various bacteria has become a clinical concern. For example, a large proportion of methicillin-resistant Staphylococcus aureus (MRSA) isolates showed resistance to quinolones (1, 2). For Streptococcus pneumoniae, a major causative organism of respiratory tract infections, the acquisition of resistance to quinolones has been reported in various areas of the world, including Southeast Asia (3). Therefore, the development of morepotent agents against resistant Gram-positive bacteria is required in preparation for the emergence of resistant strains.Lascufloxacin is a novel fluoroquinolone antibacterial agent newly developed in Japan (Fig. 1). This study was performed to determine the antibacterial activities of lascufloxacin against various clinical isolates. In addition, the activities of lascufloxacin against sequentially selected quinolone-resistant S. aureus, S. pneumoniae, and Escherichia coli were evaluated, and the inhibitory activities of lascufloxacin against target enzymes of wild-type and quinolone-resistant S. aureus were determined. Enterococcus faecalis and methicillin-susceptible S. aureus (MSSA), were limited to one isolate per patient; in some cases, two different bacterial strains, i.e., E. faecalis and MSSA, were isolated from one patient. Clinical strains of S. aureus MS5935, MS5952, MR5867, and MR6009 and their stepwise quinolone-resistant mutant strains were described previously (5). The quinolone-resistant S. pneumoniae isolates selected from strain IID553 were described previously (6). The quinolone-resistant E. coli GF4-3, CP4-1, and SP4-1 isolates were selected from K-12 strain KL-16 with gatifloxacin, ciprofloxacin,