The in vitro activities of six fluoroquinolones against 43 Brucella spp. were compared by testing three different inocula at two medium pH values. The influence of the test conditions was moderate. The activities of all quinolones were lower at pH 5 and with a high inoculum size. Results indicate the lack of effective bactericidal activity of quinolones against most strains of Brucella spp., particularly B. abortus.