Methods for introducing nucleic acids into mammalian cells are provided which use imidazolium lipids. The imidazolium lipids have the formula: wherein R1 and R2 each independently represent a C8-C24 saturated or unsaturated hydrocarbon chain, uninterrupted or interrupted by from 1 to 3 heteroatom moieties selected from —O—, —S—, —NH— and —NR—; X represents —CH2—, —O—, —S—, —NH— or —NR—; wherein R is a lower alkyl group having from 1 to 4 carbon atoms; n is an integer of from 1 to 2; and A− is an anion.