Condensed plasmid-liposome complex for transfection

An improvement in a method of preparing plasmid-liposome complexes by condensing plasmid molecules and mixing the condensed plasmids with a suspension of cationic liposomes to form plasmid-liposome complexes for use in transfecting a host cell, said improvement comprising the steps of

• selecting as a condensing agent for condensing the plasmid molecules, a polycation selected from the group consisting of histones, poly-1-glutamine, melittin and polymyxin B,
• selecting as a medium for suspending said condensed plasmid molecules, a low-ionic strength aqueous medium, and
• selecting a ratio of liposome lipid to plasmid of greater than 10 nmole liposome lipid/.mu.g plasmid and less than 15 nmole liposome lipid/.mu.g plasmid,
• where the plasmid-liposome complexes produced by the improvement have:
• (i) sizes in the range of 120-180 nm and
• (ii) a transfection stability of at least 30 days, as evidenced by a stable in vivo transfection efficiency of the complex after storage for 30 days at 4.degree. C.