The present invention provides liposomes comprising a lipid bilayer and a polymer-conjugated lipid, wherein said polymer-conjugated lipid is incorporated into said lipid bilayer. The present invention also provides methods of producing the liposomes as well as a method of delivering a nucleic acid to a subject comprising the step of administering said nucleic acid encapsulated in a mixed liposome, a method for performing diagnostic imaging in a subject, comprising the step of administering a diagnostic agent encapsulated in a mixed liposome, and methods for treating, inhibiting, or suppressing a pathological condition in a subject comprising administering to said subject a mixed liposome.