The lipid-stabilized microbubbles appear as a milky white suspension floating on the top of a layer of liquid containing unincorporated plasmid DNA. The subnatant was discarded, and the microbubbles were washed three times with PBS to remove unincorporated plasmid DNA. The mean diameter and concentration of the microbubbles in the upper layer were measured by a particle counter (Multisizer III; Beckman Coulter). The mean diameter and concentration of the microbubbles were 1.9 0.2 m and 5.2 0.3 109 bubbles per ml, respectively. The amount of plasmid carried by the microbubbles was 250 10 gml.