Most pharmaceutical and gene therapy applications of targeted liposomes Presently suffer from inefficient contents delivery to the cytoplasm of target cells. We report a plasma-stable liposome, composed of synthetic, naturally occurring diplasmenylcholine (1,2-di-O-(Z-1＇-hexadecenyl)-sn-glycero-3-phosphocholine; DPPlsC), that rapidly and efficiently releases its contents at endosomal pHs. Acid-catalyzed hydrolysis of these Liposomes produces glycerophosphocholine and fatty aldehydes, leading to greatly enhanced liposome permeability (t(50% release) congruent to 1-4 h between pH 4.5-5.5) when >20% of the vinyl ether lipid has been hydrolyzed; Plasma stability of nonhydrolyzed 9:1 DPPlsC/dihydrocholesterol liposomes exceeds 48 h at 37 degrees C, pH 7.4 in 50% serum; pure DPPlsC liposomes remain stable in 10% serum under the same conditions. Fluorescence assays of KB cells treated with 99.5:0.5 DPPlsC/DSPE-PEG3350-folate liposomes containing encapsulated propidium iodide (PI) indicate that 83% of the PI escapes the endosomal compartment within 8 h to produce intensely stained nucleii. The IC50 value of 1-beta-arabinofuranosylcytosine (Ara-C) encapsulated in DPPlsC/DSPE-PEG3350-folate liposomes is 0.49 mu M in KB cell cultures, a similar to 6000-fold enhancement in cytotoxicity compared with free drug (2.8 mM), Empty DPPlsC/DSPE-PEG3350-folate liposomes had no effect on DNA synthesis, indicating that DPPlsC and its degradation products are benign to cell function at these lipid concentrations. Our results suggest that concurrent application of selective targeting and membrane translocation mechanisms in drug carriers can significantly increase their efficacy.