In our study, the resistant mechanism of Escherichia coli to a new fluoroquinolone-HK3140 (Kim et al., 1991) was characterized. In the presence of carbonylcyanide m-chlorophenyl hydrozone (CCCP), HK3140 uptake was increased and in vivo DNA synthesis was inhibited. This result suggested that both susceptible and resistant strains have efflux systems which are proton gradient dependent. The resistant strain had another efflux system which was ATPase dependent. DNA synthesis of the permeabilized resistant cells was less inhibited by HK3140 than that of the permeabilized susceptible cells. Results suggested that the resistant mechanism against HK3140 came from an ATPase dependent efflux system and changes in DNA gyrase and porin proteins (Lee et al., 1992).