Liposomes containing PEG-substituted neutral lipopolymers provide similar circulation times to liposomes incorporating conventional, negatively charged PEG-substituted phospholipids. Use of the uncharged lipopolymers can also present advantages in terms of interactions with cell surfaces and reduced leakage of charged substances, particularly cationic drugs, from the liposomes. The lipopolymers are of the formula: wherein each of R1 and R2 is an alkyl or alkenyl chain having between about 8 to about 24 carbon atoms, n is about 10 to about 300, Z is selected from the group consisting of hydroxy, alkoxy, benzyloxy, carboxylic ester, sulfonic ester, alkyl or aryl carbonate, amino, and alkylamino, and the linkage L is selected from the group consisting of (i) —X—(C═O)—Y—CH2—, (ii) —X—(C═O)—, and (iii) —X—CH2—, wherein X and Y are independently selected from oxygen, NH, and a direct linkage.