The in vitro activity of OPC-17116 was compared to that of five similar fluoroquinolones (ciprofloxacin, enoxacin, norfloxacin, ofloxacin and temafloxacin). A total of 700 isolates from recent cases of clinical bacteremia were tested. Fifty additional stock strains with well-characterized resistance mechanisms were also processed. The minimal concentrations inhibiting 90% of strains (MIC90) of Enterobacteriaceae species were for OPC-17116 0.015-0.5 micrograms/ml and for ciprofloxacin 0.015-0.25 micrograms/ml. Moraxella catarrhalis, Haemophilus influenzae and Neisseria gonorrhoeae were very susceptible to OPC-17116 (MIC90 0.015 micrograms/ml) thus being fourfold more active than ciprofloxacin. For all beta-hemolytic streptococci and pneumococci OPC-17116 MICs were less than or equal to 0.5 micrograms/ml. The most resistant enteric bacilli were among the Citrobacter freundii and Providencia rettgeri strains (MIC90 0.5 micrograms/ml). Pseudomonas aeruginosa strains were comparably susceptible to OPC-17116 (MIC90 0.5 micrograms/ml). Low pH and CO2 incubation had an adverse effect on OPC-17116 MICs, and resistance development was documented among current clinical isolates of staphylococci, pseudomonas and some Enterobacteriaceae.