Giardia lamblia has only been grown in vitro in media containing serum or serum fractions. How this pathogen can grow in the human small intestinal lumen without serum is not known. The authors found that samples of human hepatic or gall bladder bile maintained G. lamblia survival, but not growth, for 24 to 48 hours, in medium without serum or protein. In contrast, an artificial biliary lipid (BL) dispersion containing six bile salts, phosphatidylcholine (PC), and cholesterol, in the ratios characteristic of human bile, supported parasite growth in serum and protein-free medium. To define the requirements, the authors showed that 1-palmitoyl, 2-linoleoyl-PC or 1-palmitoyl, 2-oleoyl-PC (which predominate in human bile) satisfied the requirement for PC. Moreover, either glycocholate or glycodeoxycholate could be substituted for the bile salt mixture. The finding that BL can support serum-free growth of G. lamblia may help explain why this parasite colonizes the upper small intestine. (Copyright (c) 1986, American Society for Microbiology.)