The effect of a new fluoroquinolone on the distearoyl phosphatidylcholine (DSPC) bilayers was examined above and below the phase transition temperature (T(m)) of the lipid. It was found by photon correlation spectroscopy that size and polydispersity of the extruded liposomes were unaffected by quinolone. Moreover, fluorescence quenching methods revealed a low fraction (13%) of non-accessible population of drug in the vesicles. This was interpreted in terms of encapsulation efficiency. However, variations in size correlated with decrease in the values of precipitation factor (P). These results reveal the instability of quinolone-DSPC liposomes beyond 5 days.