Plasma-stable liposomes (100 nm) were prepared from dioleoylphosphatidylethanolamine (DOPE) and 3-6 mol% of a new disulfide-linked poly(ethylene glycol)-phospholipid conjugate (mPEG-DTP-DSPE). In contrast to similar preparations containing non-cleavable PEG-phospholipid conjugate, thiolytic cleavage of the grafted polymer chains facilitated rapid and complete release of the liposome contents. Furthermore, the detachment of PEG from DOPE liposomes resulted in liposomal fusion. Finally, while formulation of pH-sensitive DOPE/cholesterol hemisuccinate liposomes with mPEG-DTP-DSPE abolished the pH sensitivity, cleavage of the PEG chains completely restored this property. These are the first examples of new useful properties of liposomes grafted with cleavable polymer.