In vitro studies were conducted to determine whether Giardia lamblia can deconjugate bile acids or inhibit lipolysis. Cultured G. lamblia trophozoites failed to deconjugate either glycine- or taurine-conjugated cholic acid, deoxycholic acid, or chenodeoxycholic acid. However, sonicated trophozoites significantly inhibited the hydrolysis of tributyrylglycerol by porcine pancreatic lipase. This inhibition was not dependent upon the presence of bile acids but was dependent upon the concentration of G. lamblia sonicate. Heating the sonicate for 20 min at 60° C abolished the inhibitory effect; dialysis of the sonicate did not affect inhibition. We conclude that cultured G. lamblia trophozoites do not deconjugate bile acids and that sonicated G. lamblia inhibit lipolysis in vitro. © 1981 Digestive Disease Systems, Inc.