PHOSPHOLIPID vesicles (liposomes1) have been used extensively as models for natural membranes1,2. Multilamellar vesicles are incorporated by mammalian cells both in vivo3,4 and in vitro 5. Sonication of such vesicles creates small (250-500 Å diameter) closed spherical vesicles composed of a single lipid bilayer enclosing an aqueous space2. Cultured mammalian cells will also incorporate these unilamellar vesicles without cytotoxic effects6. We present here data on the uptake of unilamellar vesicles by cells in vitro and the use of these vesicles as carriers to introduce biologically-active molecules into specific regions of the cell.
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