In this work, the interaction between the antibiotic ciprofloxacin (CPX) and liposomes formed with zwitterionic and acidic phospholipids was studied using fluorescence methods. Binding of 1-anilino-8naphthalene sulfonate to the liposome surface was dependent on the presence of CPX and the lipid composition. The data were fitted to a Freundlich-like isotherm. The binding constant (K), maximum concentration bound to liposomes (C-m), and cooperativity (b) were obtained. K values, in the presence and absence of CPX, were used to calculate the variation in the surface potential (Delta Psi) of the liposomes. Fluorescence quenching and anisotropy measurements suggest that the drug interacts with the headgroups of the phospholipids and does not penetrate deeper in the bilayer. No significant changes were observed in the cooperativity of the phospholipid transition. Hydrogen bonding with dip almitoylphosphatidyl-ethanolamine and electrostatic interactions with dipalmitoylphosphatidylglycerol and zwitterionic phospholipids such as dipalmitoylphosphatidylcholine appear to be involved in the interaction occurring in natural membranes.