Nucleic acid immunisation is achieved by delivering RNA encapsulated within a PEGylated liposome. The RNA encodes an immunogen of interest. The PEG has an average molecular mass above 3kDa but less than 11kDa. Thus the invention provides a liposome having a lipid bilayer encapsulating an aqueous core, wherein: (i) the lipid bilayer comprises at least one lipid which includes a polyethylene glycol moiety, such that polyethylene glycol is present on the liposome's exterior, wherein the average molecular mass of the polyethylene glycol is above 3kDa but less than 11kDa； and (ii) the aqueous core includes a RNA which encodes an immunogen. These liposomes are suitable for in vivo delivery of the RNA to a vertebrate cell and so they are useful as components in pharmaceutical compositions for immunising subjects against various diseases.
