pH-Sensitive liposomes were prepared by modifying the surfaces of egg phosphatidylcholine (EPC) liposomes and dioleoylphosphatidylethanolamine (DOPE) liposomes with an acidic proteinoid. The acidic proteinoid (Prot AL) was prepared by a melt-condensation of aspartic acid and leucine (98.5:1.5 mol/mol). The maximum amount of Prot AL accommodated by EPC liposomes without loss of the fluorescence quenching of calcein occurred when the ratio of Prot AL to EPC was 1:2. The EPC liposomes exhibited pH-dependent release but the degree of release in 5 min was less than 10% in the range of pH 6.0–8.0. A marked increase in release was observed at pH 5.5 and the degree of release was about 38%. Acidification-induced contraction of Prot AL may impose a mechanical stress on the liposomal membrane, deforming and demaging the membrane. On the other hand, a high fluorescence quenching, more than 60%, was obtained when the ratio of Prot AL to DOPE was 5.5:10. The pH-sensitivity of DOPE liposomes bearing the proteinoid was much higher than that of egg PC liposomes bearing the same proteinoid. Following the changes in the size with varying pH, DOPE liposomes seemed to be disintegrated.