With a view to prevention of liposome aggregation in plasma which is a lethal problem when using artificial erythrocytes, modification of the liposome surface with polyethylene glycol (PEG)-bound cholesterol or phospholipid was studied. The cholesterol or phospholipid moiety works as a scaffold for immobilization of the modifiers to the liposome. The modified liposome aggregability is significantly reduced with increased PEG content and particularly with increased PEG chain length. In addition, the zeta potential of modified liposomes decreases with increased PEG chain length. It is concluded that the long PEG chain exposed on the liposome surface prevents the adsorption of plasma proteins on to the surface by its excluded volume effect and, as a result, effectively reduces the liposome aggregability in plasma.