The neutral lipid and phospholipid composition of Giardia lamblia and its culture medium were analysed by a thin-layer chromatography flame-ionized detection system. Both lipid compositions of the parasite differed from that of the culture medium. Sterol was found to be the major neutral lipid, and phosphatidylcholine, phosphatidylethanolamine and sphingomyerin were also present. Fatty acid composition of G. lamblia and its culture medium were also analysed by gas liquid chromatography. Oleic and palmitic acid were the major fatty acids in the total lipid of the organism. The influence of porcine and bovine bile extracts on the lipid composition of G. lamblia was studied. The addition of bile to the medium caused no change in lipid composition of the parasite. Lipid composition of the culture medium was studied both before and following growth of the parasite, and it became evident that consumption of phosphatidylcholine occurred in the growth medium supplemented with bile extract. These results indicate that the presence of bile extract may possibly accelerate consumption by G. lamblia of lipid in the environment.