Nucleic acid immunisation is achieved by delivering RNA encapsulated within a liposome. The RNA encodes an immunogen of interest, and the liposome has a diameter in the range of 60-180nm, and ideally in the range 80-160nm. Thus the invention provides a liposome having a lipid bilayer encapsulating an aqueous core, wherein: (i) the lipid bilayer has a diameter in the range of 60-180nm 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.
