In order to characterize phospholipid liposomes prepared by reverse-phase evaporation method, the permeability of liposomes was monitored by the efflux of 6-carboxyfluorescein (6-CF). The changes of optical density with time were measured spectrophotometrically at 492 nm after gel filtration. From the simple hyperbolic curve obtained, the half-time of efflux of 6-CF was estimated as the paramter of the permeability. Half-time of efflux of phosphatidylcholine (PC) liposome (27 nmol/ml) suspended in 10 mM MES (pH 6.5) buffer with the ionic strength of 150 mM NaCl at 37℃ was 9 min. Various factors which could affect the permeability were examined including pH, temperature, ionic strength, and metal ions. These results from the PC liposome were extended to the mixed liposomes with different phospholipid head groups. The head group variation confirmed the stability of the binary membrane system. Furthermore, the selective effect of Ca^(2+) ion on the permeability of mixed liposomes was examined.