The interaction of 6-fluoroquinolones with phospholipids may account for their remarkable activity against a broad spectrum of microorganisms. In preceding papers, we have shown that ciprofloxacin (CPX) and its N-4-butylpiperazinyl derivative (BCPX) might be surface-active drugs that interact at the lipid-water interface where the headgroups reside. In this work atomic force microscopy (AFM) was applied to observe the effects of CPX and BCPX on surface planar bilayers (SPBs) formed with the phospholipid extract of membranes of Escherichia coli (E. coli). Two main features were observed when liposomes of E. coli preincubated with CPX were spread onto mica: (i) the degree of surface coverage by the liposomes was reduced in comparison with the degree covered by the liposomes nonincubated with the drug; and (ii) at high lipid-to-drug ratio patches of drug were observed which protrude from the surrounding lipid domains. BCPX, on the other hand, facilitates liposome extension onto mica. In situ experiments, carried out by injecting CPX or BCPX on SPBs, confirmed the observed behavior ex situ. CPX patches were interpreted as the result of drug self-association and the fact that this drug has a negligible partitioning into the bilayers. On the other hand BCPX, which is able to integrate into the bilayer, promotes its extension.