Minimal inhibitory concentrations (MICs) of lomefloxacin (LOM) was determined by agar dilution for 2,819 bacterial strains isolated in 1988 in 9 university hospitals. Activity of LOM against nalidixic acid (NAL) susceptible (S) Enterobacteriaceae was close to that of pefloxacin (PEF) (mode MIC: 0.12-0.25 micrograms/ml); like for PEF, this activity was reduced against NAL resistant (R) Enterobacteriaceae (mode MIC: 4). MICs of LOM against P. aeruginosa were between 0.5 and 16 (mode: 2). LOM had also a good activity against NAL S A. baumannii (mode MIC: 0.5) but this activity is reduced against NAL R Acinetobacter (MICs : 4 to 128). LOM was highly active against Haemophilus (mode MIC: 0.06), Gonococci (mode MIC: 0.008), Meningococci (mode MIC: 0.03), Branhamella (mode MIC: 0.12-0.25). LOM showed activity similar to PEF against methicillin susceptible Staphylococci (mode MIC: 0.5-1); the resistant strains are usually methicillin resistant. Similar to the currently available quinolones, LOM is less effective against L. monocytogenes (mode MIC: 8), Enterococci (mode MIC: 4), Streptococci (mode MIC: 4) and Pneumococci (mode MIC: 8). Finally, for the anaerobic bacteria, LOM is more active against Clostridium perfringens (mode MIC: 1) than against Bacteroides fragilis (mode MIC: 32).