The in-vitro antimicrobial activity of five new quinolones, enoxacin (CI-919, AT-2266), ciprofloxacin (Bay o 9867), amifloxacin (WIN 49375) norfloxacin (MK 0366), and ofloxacin (ORF 18489), was compared against 104 strains of Enterobacteriaceae, 51 Pseudomonas species, 7 Acinetobacter calcoaceticus var. anitratus, and 15 enterococci. In general the quinolones tested were active against most bacterial strains. Ciprofloxacin was the most active with MIC90 for all genera tested ranging from 0.03 to 4.0 mg/l. The range of MIC90 for enoxacin and amifloxacin was 0.06 to 16.0 mg/l, for ofloxacin 0.25 to 8.0 mg/l, and for norfloxacin 0.06 to 32.0 mg/l. All strains of P. aeruginosa tested were resistant to amikacin (MIC greater than or equal to 16 mg/l); those resistant by virtue of inactivating enzyme production were less susceptible to ciprofloxacin, ofloxacin, and norfloxacin than those strains exhibiting decreased uptake of amikacin. Susceptibility to enoxacin and amifloxacin was not affected by the mechanism of aminoglycoside resistance. Of 65 strains tested with four quinolones using log and lag phase bacterial growth, the MICs in log phase growth were one to two dilutions higher for 42% of strains tested with amifloxacin, 33% of those tested with norfloxacin, and 23% for those tested with enoxacin and ciprofloxacin.