The in vitro antibacterial activity of NM394 was compared with those of other new quinolones. NM394 showed potent and broad-spectrum antibacterial activity against 2,606 recent clinical isolates. The activity of NM394 against gram-positive bacteria was 2-to 16-fold less than that of tosufloxacin and sparfloxacin but was comparable to that of ofioxacin. Only against Streptococcus pyogenes was the activity of NM394 equal to that of sparfloxacin. Against gram-negative bacteria, NM394 showed antibacterial activity equal to that of ciprofloxacin. Against quinolone-resistant Pseudomonas aeruginosa (norfloxacin MIC, >6.25 p,g/ml), the activity of NM394 was greater than those of the other agents tested. NM394 was rapidly bactericidal at concentrations near the MIC. NM394 inhibited supercoiling activities of DNA gyrase purified from Staphylococcus aureus, Escherichia coli, and P. aeruginosa; the 50% inhibitory concentrations were 18.0, 0.41, and 2.05 ,ug/ml, respectively. (7), and OPC-17116 (3, 10), demonstrate more potent activity against gram-positive bacteria.-a]-quinoline-3-carboxylic acid} is the active form of prodrug NM441, a new fluoroquinolone antibacterial agent ( Fig. 1). In a previous report (11), the in vitro antibacterial activity of NM394 was studied in comparison with those of ciprofloxacin, ofloxacin, and enoxacin. Results indicated that NM394 had broad-spectrum antibacterial activity, including activity against gram-positive bacteria. In this study, we evaluated the in vitro activity of NM394 compared with those of other quinolones, including tosufloxacin and sparfloxacin.(Part of this work was presented previously [17]).MATERIALS AND METHODS Antimicrobial agents. NM394 was provided by Nippon Shinyaku Co., Ltd., Kyoto, and Meiji Seika Kaisya, Ltd., Tokyo, Japan. Other antimicrobial agents were provided as follows: norfloxacin, Kyorin Seiyaku Co., Ltd., Tokyo, Japan; ofloxacin, Daiichi Seiyaku Co., Ltd., Tokyo, Japan; ciprofloxacin, Bayer Yakuhin, Ltd., Osaka, Japan; tosufloxacin, Toyama Chemical Co., Ltd., Toyama, Japan; sparfloxacin, Dainippon Seiyaku Co., Ltd., Osaka, Japan; and methicillin and oxacillin, Banyu Seiyaku Co., Ltd., Tokyo, Japan.Test strains. The bacterial strains used in this study were collected between 1984 and 1991 from various hospitals in Japan and were maintained in our laboratory at -80°C in glycerol.Determination