The interaction of 6-fluoroquinolones at the lipid-water interface is the primary step for the activity of membrane transporters involved in the generation of drug resistance. In this work the interaction of the antibiotic ciprofloxacin (CPX) and its N-4-butylpiperazinyl derivative (BCPX) with dipalmitoylphosphatidylcholine (DPPC) as a model membrane is described. BCPX forms a stable film at the air-water interface and induces a condensing effect of the DPPC monolayer. A basic thermodynamic analysis was performed which suggests a possible segregation of the drug at the lowest proportion studied. The temperature dependence of 3-(4-(6-phenyl)-1,3,5-hexatrienyl)phenylpropionic acid fluorescence anisotropy measurements shows that the incorporation of CPX or BCPX into bilayers does not greatly modify the DPPC lamellar gel state. In the liquid-crystalline phase, BCPX lowered the anisotropy values and both drugs lowered the main transition temperature (Tm) of pure DPPC by approximately 1°C. On the other hand, significant changes in the cooperativity of the phospholipid transition were found only in the presence of BCPX. In 31P-NMR experiments the presence of BCPX induced, both below and above the main transition, a strong line shape narrowing of the DPPC spectra that suggests an increase in the local mobility of the phosphate groups; that is, the interactions between DPPC headgroups are altered by the presence of BCPX. Although the incorporation of CPX also promotes a remarkable line shape narrowing in the fluid phase, it only slightly modifies the spectral parameters of DPPC in the gel phase. This suggests that CPX could be "squeezed out" or segregated from the lipid-water interface when the phospholipids are in this tight packed phase. The segregation is likely to occur to defects on the gel phase. Values of 1-anilino-8-naphthalenesulfonate binding to the liposome surface were fitted to a Freundlich-like isotherm. The binding constant (K) and maximum concentration bound to liposomes (Cm) both are dependent on the structure of the drugs, which indicates a definite effect due to the drug hydrophobicity. K values in the presence and absence of drug were used to calculate the variation in the surface potential (ΔΨ) of the liposomes. All the results are consistent with an electrostatic interaction of 6-fluoroquinolones at the lipid-water interface. This interaction is favored by the presence of the N-4-butyl chain and could have important implications in the efflux of this drug from bacteria.