The physical characteristics of polymerized liposomes for potential use as an oral drug delivery system were examined in vitro. The trap efficiency in monomeric liposomes composed of 1,2-di (2,4-octadecadienoyl) phosphatidylcholine was increased from 3% for original multilamellar vesicles to 35% for freeze-thaw treated liposomes. Polymerized liposomes with azobis (isobutyronitrile) and azobis (2-amidinopropane) hydrochloride as radical initiators showed complete stability against solubilization by Triton X-100, a detergent chosen to mimic bile salts. Release rates of 14C-BSA and 14C-sucrose in media simulating the gastro-intestinal fluids was 50% less than from regular liposomes composed of hydrogenated egg phosphatidylcholine mixed with cholesterol (molar ratio 1:1), which can be regarded as one of the most stable types of regular liposomes. It was estimated that, when administered orally, polymerized liposomes can reach the intestine while maintaining their vesicle structure and keeping at least 75% of their original content.