1. Post-heparin lipolytic activity in man has been studied by using a triglyceride substrate emulsion containing different emulsifiers. 2. The lipolytic activity measured was profoundly influenced by the type of emulsifier used in the substrate. Substrate stabilized by synthetic emulsifiers give higher lipolytic activity than Intralipid, which contains egg phospholipids as emulsifiers. This difference was solely explained by higher salt-resistant lipase activities found with emulsions containing synthetic emulsifiers. The salt-inhibited lipase activity, which has properties as a lipoprotein lipase, was not influenced by the type of emulsifier. 3. When used under specified conditions Intralipid seems to be virtually specific for extrahepatic post-heparin lipolytic activity.