In vitro antibacterial activities (MBCs) and serum bactericidal activities (SBAs) of fluoroquinolones were compared. The MBCs were measured by the agar disk dilution test. The 40 tested strains belonging to 8 species were isolated from hospitalized patients. Against Gram positive cocci, Ciprofloxacin (CPLX) and ofloxacin (OFLX) were the more active and nefloxacin (NFLX) was less potent than the others. Against Ps. aeruginosa, CPLX had the highest activity, enoxacin (ENX) as well as OFLX had powerful activity, whereas NFLX was less active. All the 4 agents showed high activity against Gram negative bacilli. In the self-controlled, randomized crossover study, 4 fluoroquinolones were given to 10 healthy volunteers and then SBAs were determined using microdilution method. The peak SBAs of CPLX and OFLX were significantly higher than NFLX, and those of ENX were comparatively low, but there was no difference between ENX and OFLX against most of the strains tested. The percentages of peak SBAs greater than 1:8 of the 4 fluoroquinolones suggest that in the treatment of serious infections, CPLX and OFLX are more effective and ENX can also achieve high cure rate. The trough SBAs of the 4 fluoroquinolones suggest that the time interval of administration of CPLX and OFLX should be more than 8 hours, but increase of the dosage or shortening of the time interval between the administrations is recommended for ENX and NFLX.