The in vitro activity of amifloxacin, a quinolone antimicrobial agent was compared with those of ciprofloxacin, enoxacin, ofloxacin and norfloxacin against gram-positive and gram-negative bacteria. Ninety percent of Escherichia coli, Klebsiella species, Aeromonas, Salmonella, Shigella, Citrobacter, Enterobacter species, Proteus mirabilis, Serratia marcescens, and Morganella morganii were inhibited by less than or equal to 0.5 microgram/ml. Amifloxacin inhibited Branhamella, Haemophilus, and Neisseria at less than or equal to 0.25 microgram/ml, and 90% of Pseudomonas aeruginosa, including gentamicin- and carbenicillin-resistant isolates, at 4 micrograms/ml. It also inhibited staphylococci, including methicillin-resistant isolates, but was less active against streptococci and Bacteroides species. Amifloxacin had in vitro activity similar to enoxacin, ofloxacin, and norfloxacin, but was less active than ciprofloxacin. Like other quinolones, it was less active at acid pH and in the presence of urine.