The search for alternative therapeutic agents for listeriosis includes the quinolone group. Accordingly, the bactericidal activity of ciprofloxacin, levofloxacin, lomefloxacin, ofloxacin, sparfloxacin, and temofloxacin, in comparison with that of ampicillin and sulfamethoxazole-trimethoprim, was evaluated against Listeria monocytogenes at 24 and 48 h of incubation using time-kill kinetic methodology. The inhibitory concentrations for each agent fell into a narrow range comparable with ampicillin. For example, the minimum inhibitory concentration (MIC) ranges, MIC90 (24 h), and MIC90 (48 h) of the most active quinolone, sparfloxacin, were 0.25-2, 2, and 2 micrograms/ml, respectively, with 4 micrograms/ml achieving > or = 99.9% killing of the inoculum at 24 h with no regrowth by 48 h. At 2-4 times the MIC, bactericidal activity for all quinolones tested was noted at 24 h, unlike the action of ampicillin, which only becomes bactericidal at 48 h. These concentrations are within the achievable range of serum concentrations for a number of these agents. Because selected new fluoroquinolones at two to four times the MIC show bactericidal activity at 24 h, these agents may prove useful as therapeutic alternatives for the treatment of listeriosis.