Ofloxacin is a new quinolone carboxylic acid compound. Its activity against 900 bacterial isolates was determined. It inhibited 90% of Escherichia coli, Klebsiella sp., Aeromonas hydrophila, Salmonella spp., Shigella spp., Citrobacter spp., Enterobacter spp., Morganella morganii, Proteus mirabilis, Yersinia enterocolitica at less than or equal to 0.8 mg/l. Branhamella catarrhalis, Haemophilus sp., Neisseria sp. were inhibited by less than or equal to 0.1 mg/l. Pseudomonas aeruginosa and other Pseudomonas species were inhibited by less than or equal to 6.3 mg/l. Although most staphylococcal species, including methicillin-resistant staphylococci were inhibited by 3.1 mg/l, many streptococcal species had higher MIC values, and most Bacteroides species were inhibited at less than or equal to 6.3 mg/l. The overall activity of ofloxacin was similar to enoxacin and norfloxacin. Ofloxacin inhibited organisms resistant to nalidixic acid, ampicillin, cephalexin, piperacillin, and gentamicin including Enterobacter spp., Citrobacter freundii and Serratia marcescens resistant to cefotaxime. The activity of ofloxacin was lower at an acid pH and in urine, but serum had no effect on MICs or MBCs. Increase in ofloxacin MICs for various bacteria could be achieved by repeated subculture in the presence of ofloxacin.