Abstract: 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 of between 1 kDa and 3 kDa. 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 between 1 kDa and 3 kDa; 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.